That CPU is 12th gen and shouldn’t have too much trouble transcoding but I would plan the build around being able to add GPU if necessary. So be careful with mini ITX. I’m not sure about your internet speed but 4K movies that are ~50gb each will need 30-50mbps upload speed per active stream to play without transcoding.

I am going to have the Define 7 XL case for a long, long time. It is built like a tank.