Yeah I think that's the whole difference.

For single-player, Quake 4 felt more related to Doom 3 and the later Doom remakes. A more modern, atmospheric, FPS dungeon crawler.

For me, Quake 3 was the end of my appreciation for Id games, since I am only interested in single-player and it didn't even try to offer that. I am drawn to the immersive FPS environment, but I have no competitive urge nor enjoyment from having random participants disrupting the experience.