I wonder if I'm imagining it but the keyboard on the M4 Air feels better than the one on the M2 Air. Travel feels "better" (I can't pinpoint how to describe it though) and the keys feel less slippery but that might just be how a clean, new keyboard feels.
I could not find anything in reviews about the keyboard being improved other than that the M4 Air keyboard has a strike-through on the speaker icon on the F10 key now.