The answer will not change if you ask it on a different forum.
You'll need at least an RPi-4B if you want to process/watch 4K videos.
Or other 4K capable hardware.
I know that the answer will not change. But I think that there's no problem if I ask for the same problem on two different forums.
BTW, the 3B+ didn't support 4K, ok, but I supposed that wasn't a problem because my TV is only full-hd, so I expected that the 3B+ will reproduce it downscaled only at 1080P.
I apologize if this supposition was wrong.