What games actually use 4 buttons for player 3 and 4?
You can find answers to these types of questions at MAWS (use the 'deluxe search'):
http://www.mameworld.net/maws/Here's what MAWS spits out for 4 simultaneous players and 4 buttons each (1st of 3 pages):
arabiamu Arabian Magic (Ver 1.0A 1992/07/06) Taito America Corporation 1992 .037b11 Fighter / 2.5D
arabiamj Arabian Magic (Ver 1.0J 1992/07/06) Taito Corporation 1992 .037b11 Fighter / 2.5D
arabianm Arabian Magic (Ver 1.0O 1992/07/06) Taito Corporation Japan 1992 .037b11 Fighter / 2.5D
blzntrnd Blazing Tornado Human Amusement 1994 .037b13 Sports / Wrestling
cyclwarr Cycle Warriors Tatsumi 1991 .087u1 Fighter / Misc.
dungenmu Dungeon Magic (Ver 2.1A 1994/02/18) Taito America Corporation 1993 .061 Maze / Fighter
dungeonm Dungeon Magic (Ver 2.1O 1994/02/18) Taito Corporation Japan 1993 .061 Maze / Fighter
ddsoma Dungeons & Dragons: Shadow over Mystara (Asia 960619) Capcom 1996 .055 Fighter / 2.5D
ddsomr3 Dungeons & Dragons: Shadow over Mystara (Euro 960208) Capcom 1996 .106u10 Fighter / 2.5D
ddsomr2 Dungeons & Dragons: Shadow over Mystara (Euro 960209) Capcom 1996 .099u07 Fighter / 2.5D
ddsomr1 Dungeons & Dragons: Shadow over Mystara (Euro 960223) Capcom 1996 .055 Fighter / 2.5D
ddsom Dungeons & Dragons: Shadow over Mystara (Euro 960619) Capcom 1996 .071u2 Fighter / 2.5D
ddsomjr1 Dungeons & Dragons: Shadow over Mystara (Japan 960206) Capcom 1996 .037b15 Fighter / 2.5D
ddsomj Dungeons & Dragons: Shadow over Mystara (Japan 960619) Capcom 1996 .053 Fighter / 2.5D
ddsomur1 Dungeons & Dragons: Shadow over Mystara (USA 960209) Capcom 1996 .037b15 Fighter / 2.5D
ddsomu Dungeons & Dragons: Shadow over Mystara (USA 960619) Capcom 1996 .059 Fighter / 2.5D
ddtoda Dungeons & Dragons: Tower of Doom (Asia 940113) Capcom 1993 .037b12 Fighter / 2.5D
ddtodr1 Dungeons & Dragons: Tower of Doom (Euro 940113) Capcom 1993 .096u3 Fighter / 2.5D
ddtod Dungeons & Dragons: Tower of Doom (Euro 940412) Capcom 1993 .056 Fighter / 2.5D
ddtodh Dungeons & Dragons: Tower of Doom (Hispanic 940125) Capcom 1993 .059 Fighter / 2.5D
ddtodjr1 Dungeons & Dragons: Tower of Doom (Japan 940113) Capcom 1993 .037b12 Fighter / 2.5D
ddtodj Dungeons & Dragons: Tower of Doom (Japan 940125) Capcom 1993 .086u2 Fighter / 2.5D
ddtodur1 Dungeons & Dragons: Tower of Doom (USA 940113) Capcom 1993 .037b12 Fighter / 2.5D
ddtodu Dungeons & Dragons: Tower of Doom (USA 940125) Capcom 1993 .037b12 Fighter / 2.5D
hthero93 Hat Trick Hero '93 (Ver 1.0J 1993/02/28) Taito Corporation 1993 .037b11 Sports / Soccer