Question about Raspberry Pi 3 Model B

Not really, no. Frame rate at 4K resolution will not be good, you can forget about transcoding. 1080@60 already brings it to the limit, so even if you clock it high, the RPi 3 can't deliver a usable framerate at 4K.

The recently released ASUS Tinker Board (not yet available at Digitec) has the same dimensions and connections as the Rasberry Pi 2/3/B, but is supposed to be explicitly suitable for 4K. However, it doesn't seem to be available anywhere yet, so the test reports are also limited.