I've managed to plug 5 controllers via Bluetooth but never gone beyond that. Honestly donno if we noticed any lag or whatever, but this was on an older pi3b, which I think has weaker Bluetooth.
Depends how fussy people are, if they just want to play bomberman and have a laugh, I doubt they'd notice the lag. If they're hardcore online gamer types, maybe go for usb controllers and a hub.
In terms of games, the pc engine has capability for 5 player, but it's just bomberman which you've already got covered.
Look for a micro machines j-cart ROM on megadrive, that's 4 player I think. Also arcade beat em ups are fantastic 4 player, Simpsons, turtles, X men (6 player!), There's tons of them but I can't remember them all.
Edit:- just use this list for games btw, it's pretty exhaustive:- https://retropie.org.uk/forum/topic/4336/list-of-retro-games-for-4-players