A Commodore 64 only had 2 joystick ports, so the C64 version of M.U.L.E. uses the keyboard for players 3 and 4 on screens where all players are active, but accepts input from any joystick on screens where only one player is active -- allowing players to pass the sticks around to take their "turns".

The Atari version of the game supports four joysticks as did the underlying hardware. This is the version you want to play in emulation if you have 4 players. Great fun!