First, I suggest making just one loop and check all buttons inside of it.
You need to link players with their gamepads. You can add an instance variable GamepadNo to the player sprite. When a button is pressed on gamepad #1, pick a player with GamepadNo=1 and control it.
Or you can do this with an array, for example X-index of the array is the player number, and the value is gamepad number. So for 4 players your array may look like this:
You fill this array at the start of the game, when you ask each player to press a button on their gamepads. Then in the game when a button is pressed on gamepad with index 1, you know that it belongs to player #3.