Guilty Gear Isuka uses 5.
It's playable with Demul, but not worth the hassle of the extra buttons IMO.
If I were building a 4 player panel, I'd probably only use 3 buttons for P3 & P4.
Most people who put more buttons for P3 & P4 are doing it for console emulators.
(N64, Dreamcast, Gamecube)
See if there are any games for those systems you'd like to be able to play with 4 players,
then see how many buttons the games actually require.
(think Mario Party, Super Smash Bros, & sports games)