commercially available 4k contents are almost non-existent. Its sad considering these facts
1) 4k hardware are maturing with arrival of HDMI 2.0 and HDCP2.2
2) contents were available since donkeys years ago as all film delivered by digital theatre has been in native 4k (albeit 4096, not 3840) formats.
Problem however lies within delivery technology:
1) BD disc was not designed with 4k in mind. 4k BD standard has been very slow to arrive, let alone adopted. So called 4k BD player in market does upscaling, which arguably if you have a good set of 4k TV, the TV does the same if not better upscaling job than your BD player.
2) VP9/VP10/HEVC H.265 has only been recently standardized. Almost none of 2014 4k model support them so if you have one of the 2014 sets, you can kiss 4k streaming straight from your TV goodbye.
3) content distributor has been very paranoid about 4k. Despite them having a native master copy of 4k films all these while, these are considered as the holy grail contents that differentiate a cinema experience from your home cinema. And if these contents managed to leak all over the internet like 1080p contents, they are afraid cinema goers would stay home. Even 4k Netflix has been very tightly controlled with very specific hardware certification to ensure this does not happen. Arrival of HDCP 2.2 might convince them to change though.
Last but not least for where to get 4k contents:
1) YouTube 4k - nothing interesting here. A couple of good short film which is good for demo. Those coming from GoPro / action camera can be skipped right away the camera compression + youtube compression makes them less shiny than actual properly done 4k videos.
2) Netflix 4k TV - Good contents although limited, they are expanding the catalog at a steadier pace compared to others. Tricky part is they have very tightly controlled hardware that so far only seen a couple of the biggest name in TV hardware managed to get their TV certified to stream Netflix 4k. And these feature has been removed for overseas set hence for example the feature is disabled in X8500C despite being present in US X8500B. And no, you can't stream netflix 4k over the web.
3) Netflix 4k certified hardware - Has to fullfill all content protection set by Netflix and very little present at the moment, in fact only one worth mentioning, namely nVidia Shield TV. And at USD 199, it is arguably very affordable too considering Sony is charging thousands for its FMP-X.
4) Amazon 4k / others - mostly are priced per movie / episode. Not very economical for me considering Netflix is charging USD 12 per month for unlimited access.
