We tried the pi4 for a project where we had to playback 4-6 videos simultaneously. As far as I could tell, it was properly configured for hardware decoding. It bogged down noticeably after 3 videos and was barely capable of doing 4. On paper it looked great; in real life not so much.
Ended up going with a NUC for that part of the project and it worked great. The other aspects of the project that didn't need video (we had close to a dozen total displays), the pi did just fine.
The issue trackers mentions a few attempts at getting 1080p streams running, but there definitely seems to be a limitation of some sort with more than 3 higher resolution videos.
We started out trying to do 4k and when that didn't work, dropped back to 1080P. Looked good with just one or two, but above that it got "stuttery." 6 videos on the screen was absolutely not possible with the pi.
Ended up going with a NUC for that part of the project and it worked great. The other aspects of the project that didn't need video (we had close to a dozen total displays), the pi did just fine.