4K resolution , also called 4K , refers to a horizontal screen display resolution in the order of 4,000 pixels.  There are several different 4K resolutions in the fields of digital television and digital cinematography. In television and consumer media, 4K UHD or UHD-1 is the dominant 4K standard. In the movie projection industry, Digital Cinema Initiatives (DCI 4K) is the dominant 4K standard.
In 2015, the 4K television market share increased as prices fell dramatically during 2014  and 2015. By 2020, more than half of U.S. households are expected to have a 4K-capable TVs (2160p)  , which would be a much faster adoption rate than that of Full HD ( 1080p). 
There are three main 4K resolution standards:
- UHD-1 , or ultra-high-definition television (UHDTV), is the 4K standard for television and computer monitors. UHD-1 is also called 2160p   as it has a resolution of 3840 × 2160 ( 16:9, or approximately a 1.78:1 aspect ratio), which is twice the horizontal and twice the vertical resolution of 1080p, or three times the horizontal and vertical resolution of 720p.  UHD-1 is used in consumer television and other media, e.g. video games.
- UW4K is the ultra-wide 4K standard, with a resolution of 3840 × 1600, and an aspect ratio of 12:5 (2.4:1, or 21.6:9) This resolution is most commonly used on Ultra HD Blu-ray discs, and PC gaming monitors.
- DCI 4K which has a resolution of 4096 × 2160 pixels (256:135, approximately a 1.9:1 aspect ratio). This standard is only used in the film and video production industry.  The DCI 4K standard has twice the horizontal and twice the vertical resolution of DCI 2K.
Many manufacturers may advertise their products as UHD 4K, or simply 4K, when the term 4K is traditionally reserved for the cinematic, DCI resolution.    This has the potential to cause confusion for consumers. 
YouTube and the television industry have adopted UHD-1 as their 4K standard.   As of 2014, 4K content from major broadcasters remains limited.  On April 11, 2013, Bulb TV created by Canadian serial entrepreneur Evan Kosiner became the first broadcaster to provide a 4K linear channel and VOD content to cable and satellite companies in North America.     The channel is licensed by the Canadian Radio-Television and Telecommunications Commission to provide educational content.  However, 4K content is becoming more widely available online including on YouTube, Netflix and Amazon.  By 2013, some UHDTV models were available to general consumers in the range of US$600.  As of 2015, prices on smaller computer and television panels had dropped below US$400.  DVB expects UHD-1 Phase 2 services to be introduced by broadcasters from 2017, with features such as High Dynamic Range (using HLG and PQ at 10 or 12 bits), Wide Color Gamut ( BT. 2020 / 2100 colorimetry), and High Frame Rate (up to 120 Hz). 
The first commercially available 4K camera for cinematographic purposes was the Dalsa Origin, released in 2003.   YouTube began supporting 4K for video uploads in 2010 as a result of leading manufacturers producing 4K cameras.  Users could view 4K video by selecting "Original" from the quality settings until December 2013, when the 2160p option appeared in the quality menu.  In November 2013, YouTube started to use the VP9 video compression standard, saying that it was more suitable for 4K than High Efficiency Video Coding (HEVC); VP9 is being developed by Google which owns YouTube. 
Sony is one of the leading studios promoting UHDTV content, as of 2013 offering a little over 70 movie and television titles via digital download to a specialized player that stores and decodes the video. The large files (~40GB), distributed through consumer broadband connections, raise concerns about data caps. 
In 2014, Netflix began streaming House of Cards , Breaking Bad ,  and "some nature documentaries" at 4K to compatible televisions with an HEVC decoder. Most 4K televisions sold in 2013 did not natively support HEVC, with most major manufacturers announcing support in 2014.  Amazon Studios began shooting their full-length original series and new pilots with 4K resolution in 2014.  They are now currently available though Amazon Video. 
In 2016, Sony and Microsoft released the PlayStation 4 Pro and Xbox One S, respectively, both of which are video game consoles that support 4K streaming and gaming; the Xbox One S also features an Ultra HD Blu-ray disc drive. On November 7, 2017 Microsoft released the Xbox One X, which is also capable of 4K streaming and gaming. 
Home video projection
Though experiencing rapid price drops beginning in 2013 for viewing devices, the home cinema digital video projector market saw little expansion, with only a few manufacturers (only Sony as of 2015) offering limited 4K-capable lineups, with native 4K projectors commanding five-figure price tags well into 2015 before finally breaking the US$10,000 barrier.  Critics state that at normal direct-view panel size and viewing distances, the extra pixels of 4K are redundant at the ability of normal human vision. Whereas, projection home cinemas employ much larger screen sizes without necessarily increasing viewing distance to scale. JVC has used a technique known as "e-shift" to extrapolate extra pixels from 1080p sources to display 4K on screens through upscaling or from native 4K sources at a much lower price than native 4K projectors.  This technology of non-native 4K entered its fourth generation for 2016.    JVC used this same technology to provide 8K flight simulation for Boeing that met the limits of 20/25 visual acuity. 
Pixel shifting as described here, was pioneered in consumer space by JVC, and later, in commercial space by Epson. That said, it isn't the same thing as "true" 4K. More recently we now have DLP projectors claiming 4K UHD (which the JVCs and Epsons do not even attempt to claim). Here's what need to know, to understand what's going on, and what's really best:
First, to confirm DCI (movie theaters) is 4096×2160, while "true" 4K is 3840×2160, talking a slight difference in aspect ratio, rather than a significant difference in resolution. In traditional displays, such as LCD or OLED, talking 3840 pixels across the screen - with each pixel being 1/3840th of the screen width. They do not overlap - if they did, detail would be reduced. The diameter of each is basically 1/3840th of the screen width or 1/2160th of the screen height - either gives the same size pixel.
That 3840×2160 works out to 8.3 megapixels, the official resolution of 4K UHD (and therefore Blu-ray UHD discs).
But the 4K UHD standard doesn't seem to care how large the pixels are, so a 4K UHD projector (Optoma, BenQ, Dell, others) counts, because these projectors have a 2718×1528 pixel structure. Those projectors process the true 4K of data, and figure out the best way to handle it with overlapping pixels, which is what pixel shifting is all about. Unfortunately, each of those pixels is far larger, each one has 50% more area than true 4K. Those pixel shifting projectors fire a pixel, shift it up to the right, by a half diameter, and fire it again, with modified data, but that second firing overlaps the first.
In other words pixel shifting is not capable of producing adjacent vertical lines of RGBRGB or any other colors where each line is pixel (1/3840th) wide. Adjacent red and green pixels would end up looking like yellow. with a fringe on one side of red, on the other of green - except that the next line of pixels will be overlapping as well, changing the color of that fringe. Simply stated, there is no way 4K UHD, or 1080p pixel shifting can reveal the fine detail of a true 4K projector such as those Sony ships (business, education and home markets). Also JVC has one true 4K projector priced $35,000 (as of mid-2017).
So while 4K UHD sounds like it was going to have pixel structures with 1/4 the area of 1080p, that's just not going to happen with pixel shifting.
Only a true 4K projector will offer that level of resolution. That should help explain why "true" 4K projectors cost so much more than 4K UHD projectors with otherwise similar feature sets. Getting smaller pixel, finer resolution, no compromising of detail or color from overlapping pixels.
By comparison the slight difference in aspect ratio between DCI and 3840×2160 pixel displays without overlap is insignificant relative to the amount of detail that can be seen.
In November 2014, United States satellite provider DirecTV became the first pay TV provider to offer access to 4K content, although limited to selected video-on-demand films.  In August 2015, British sports network BT Sport launched a 4K feed, with its first broadcast being the 2015 FA Community Shield football match. Two production units were used, producing the traditional broadcast in high-definition, and a separate 4K broadcast. As the network did not want to mix 4K footage with upconverted HD footage, this telecast did not feature traditional studio segments at pre-game or half-time, but those hosted from the stadium by the match commentators using a 4K camera. BT envisioned that if viewers wanted to watch studio analysis, they would switch to the HD broadcast and then back for the game. Footage was compressed using H.264 encoders and transmitted to BT Tower, where it was then transmitted back to BT Sport studios and decompressed for distribution, via 4K-compatible BT TV set-top boxes on an eligible BT Infinity internet plan with at least a 25 Mbit/s connection.  
In late 2015 and January 2016, three Canadian television providers – including Quebec -based Videotron, Ontario-based Rogers Cable, and Bell Fibe TV, announced that they would begin to offer 4K compatible set-top boxes that can stream 4K content to subscribers over gigabit internet service. On October 5, 2015, alongside the announcement of its 4K set-top box and gigabit internet, Canadian media conglomerate Rogers Communications announced that it planned to produce 101 sports telecasts in 4K in 2016 via its Sportsnet division, including all Toronto Blue Jays home games, and "marquee" National Hockey League games beginning in January 2016. Bell Media announced via its TSN division a slate of 4K telecasts to begin on January 20, 2016, including selected Toronto Raptors games and regional NHL games.  
On January 14, 2016, in cooperation with BT Sport, Sportsnet broadcast the first ever NBA game produced in 4K – a Toronto Raptors / Orlando Magic game at O2 Arena in London, England. On January 20, also during a Raptors game, TSN presented the first live 4K telecast produced in North America.   Three days later, Sportsnet presented the first NHL game in 4K. 
Dome Productions, a joint venture of Bell Media and Rogers Media (the respective owners of TSN and Sportsnet), constructed a "side-by-side" 4K mobile production unit shared by Sportsnet and TSN's first 4K telecasts; it was designed to operate alongside a separate HD truck and utilize cameras capable of output in both formats.  For the opening game of the 2016 Toronto Blue Jays season, Dome constructed "Trillium" – a production truck integrating both 4K and 1080i high-definition units.  Bell Media's CTV also broadcast the 2016 Juno Awards in 4K as the first awards show presented in the format. 
In February 2016, Univision trialed 4K by producing a closed circuit telecast of a football friendly between the national teams of Mexico and Senegal from Miami in the format. The broadcast was streamed privately to several special viewing locations. Univision aimed to develop a 4K streaming app to publicly televise the final of Copa América Centenario in 4K.   In March 2016, DirecTV and CBS Sports announced that they would produce the "Amen Corner" supplemental coverage from the Masters golf tournament in 4K. 
|Format||Resolution||Display aspect ratio||Pixels|
|Ultra-high-definition television||3840 × 2160||1.78:1 ( 16:9)||8,294,400|
|Ultra-wide-television||3840 × 1600||2.4:1 (12:5)||6,144,000|
|DCI 4K ( native resolution)||4096 × 2160||1.90:1 (256:135)||8,847,360|
|DCI 4K ( CinemaScope cropped)||4096 × 1714||2.39:1 (1024:429)||7,020,544|
|DCI 4K ( flat cropped)||3996 × 2160||1.85:1 (999:540)||8,631,360|
4K UHD (2160p) a resolution of 3840 pixels × 2160 lines (8.3 megapixels, aspect ratio 16:9) and is one of the two resolutions of ultra high definition television targeted towards consumer television, the other being 8K UHD which is 7680 pixels × 4320 lines (33.2 megapixels). 4K UHD has twice the horizontal and vertical resolution of the 1080p HDTV format, with four times as many pixels overall.   Likewise, 4K UHD has three times the horizontal and vertical resolution of the 720p format, with nine times as many pixels overall.
The Digital Cinema Initiatives consortium established a standard resolution of 4096 pixels × 2160 lines (8.8 megapixels, aspect ratio 256:135) for 4K movie projection. This is the native resolution for DCI-compliant 4K digital projectors and monitors ; pixels are cropped from the top or sides depending on the aspect ratio of the content being projected. The DCI 4K standard has twice the horizontal and vertical resolution of DCI 2K, with four times as many pixels overall. DCI 4K does not conform to the 16:9 aspect ratio, so it is not a multiple of the 1080p display.
4K digital movies may be produced, scanned, or stored in a number of other resolutions depending on what storage aspect ratio is used.  In the digital cinema production chain, a resolution of 4096 × 3112 is often used for acquiring "open gate" or anamorphic input material, a resolution based on the historical resolution of scanned Super 35mm film. 
YouTube, since 2010,  and Vimeo allow a maximum upload resolution of 4096 × 3072 pixels (12.6 megapixels, aspect ratio 4:3).   Vimeo's 4K content is currently limited to mostly nature documentaries and tech coverage.  
YouTube added support for up to 8K 7680×4320 video content in June 2015.
The main advantage of recording video at the 4K standard is that fine spatial detail is resolved well.  If the final video quality is reduced to 2K from a 4K recording more detail is apparent than would have been achieved from a 2K recording.  Increased fineness and contrast is then possible with output to DVD and Blu-ray.  Some cinematographers choose to record at 4K when using the Super 35 film format to offset any resolution loss which may occur during video processing.