Here are my requirements for a HTPC, or a device I will hook up to my TV and watch Movies/TV Shows with:
So far the device that suits me the best is the Raspberry Pi. Model 2 in my case. It does all of these things except 4K and navigate OpenPHT without lag. The lag isn't bad mind you, but it's a bit annoying. Just a sliver. Apparently the speed difference is noticable with a Model 3 so I could potentially just get one of those and it would be perfect, except for 4K.
But instead of spending money on yet another Raspberry Pi I'd rather save it for a Nvidia Shield TV or something nice like that.
The Raspberry Pi 2 is great though, it's just slow sometimes. If you are just binge watching a show, i.e. you just navigate to the next episode and hit play (2 clicks on the remote) it's totally fine. It's just slow when you are scrolling through 1000 movies or 300 TV shows.