I’m not sure which ones he was into at 4yo specifically, but my son’s Switch favorites include…

Super Mario Odyssey Mario + Rabbids: Kingdom Battle Celeste Minecraft Yoshi’s Crafted World Letterquest Big Brain Academy NES Arcade SNES Arcade

Of those, the ones I would say mught meet your super-chill criteria are…

Super Mario Odyssey (yeah, you can die, but you just respawn and can spend tons of time just running around aimlessly) Celeste (normally not, but there’s a lovely Assist Mode) Yoshi’s Crafted World (there’s a no-fail mode) Big Brain Academy (if they can handle being scored on things, without taking it too seriously).