I wonder if I'm doing something wrong. I have a 4GiB pi 4B and have found it laggy enough to be basically completely unsuitable for even the most lightweight interactive computing tasks, e.g. basic (ad filtered) web browsing. I think there might have been a bit of intermittent laginess in typing into an xterm in a bare bones tiling window manager with basically nothing else going on, but I'd have to check again. After the initial hickup on full-screening, did the video play back properly for you? For me it was not possible to play 480p youtube videos.
All in all this was much worse than I expected from a quad core machine with 4GiB.
Now I haven't got the fastest micro-SD card, and I'm running with Ubuntu 20.04 64bit server rather than raspian, but is this just what I should expect, or should a different configuration of the 4GiB 4B handle light web surfing and fullscreen video playback?