Great news. I'm glad to see the ps4 will be ready for 4K movies. I've already been looking to upgrade my 1080p projector but so far there is only 1 4k projector available, still 'a bit' out of my price range at 25k :/
http://www.trustedreviews.com/sony-vpl-vw1000es_Projector_review
Maybe some arcade games will be using 4K as well, a 4K picture viewer will be awesome too.
Whether you can use it all depends on how big a field of view you want. If you want to enjoy movies at the optimal THX recommended distance (1.2 x diagonal screen size) then 2160p is very useful.
Check my thread if you want to see what resolution and size you can use.
http://gamrconnect.vgchartz.com/thread.php?id=146412
@Selnor Do you think your digital downloads will be upgraded to 4K for free? I rather hang on to a backwards compatible dvd / blu-ray collection then trusting a server.