Yes.
Yes, as long as the content is in the format Apple uses for 4K, it should play fine. Many people have libraries of videos indexed in iTunes and readily available on their

TV right now. It's a terrific little box for video collection on demand.
I've had every

TV generation since launch. It's always worked very well. The sole gripe I have is that Apple tends to cling to a max "HD" resolution longer than pretty much everyone else. For example, until the launch of

TV3, Apple clung to 720p as the max HD resolution.
And of course, many of the crowd here passionately argued that 1080p was a gimmick, until the iTunes store was full of 1080p content for

TV, 1080p TVs are too expensive, "the chart", until bandwidth everywhere was expanded, file sizes, and the good old "I can't see the difference... (so you can't either), etc right up until Apple rolled out the "3" and then all such arguments evaporated.
For the many years since, 1080p has been all anyone should want and 4K is the gimmick, "until the iTunes store is full of 4K content for

TV, "4K TVs are too expensive, "the chart" until bandwidth everywhere is expanded, file sizes and the good old "I can't see the difference... (so you can't either)", etc. Prepare for all that to evaporate too as soon as Apple rolls out a 4K-capable

TV5. Same move... different resolution.
3 or 4 years from now, recycle the very same arguments yet again for 8K.