phantom breaker battlegrounds Ultimate, it's freaking wild, crazyiest beat em up i've ever played
4 players, and 8 online, so many characters as shown below, that's basically all enemies and bosses
River city girls 2, it's 4 players, i believe 8 characters, it's good fun
(not shown, double dragons billy and jimmy as DLC)
Ninja turtles games: shredders revenge is 6 players (i think local, online is def 6)
on switch, kirby star allies is 4 players and has a lot of playable characters, it's fun with 4 players definitely, it's beat em up enough in my opinion lol
check out kirby return to dreamland Deluxe, also 4 players but not quite a selection as this
other games that should be very fun in 4 players:
Rayman legends (ridiculous amount of levels it'll take a long time to 100%)
castle crashers
Guacamelee (i believe both have 4 player co-op on switch) this is beat-em-up metroidvania at it's best.
streets of rage 4, keep in mind theres a lot of unlockable characters (and i think one dlc)