I use a RPI 5 for this purpose but it's hard to recommend it at this point. Some videos play just fine but others struggle and drop frames. Seems totally random at times. I can promise though that 4k will not play watchably. 1080p is the max you can do and like I said, even that can be iffy. It was mostly a fun project to setup but also annoying at times due to concerns with ARM compatibility, but I really wish the performance was better. If I could smoothly watch 4k without being tracked all of it would've been worth it and then some.

Edit: if anyone has a good recc for a power efficient and budget friendly mini pc, I'd love that myself.