Just figured I'd post this up as a good reference.
Player 4 LED flashes 1x - Left thumbstick
Player 4 LED flashes 2x - Left bumper
Player 4 LED flashes 3x - Right bumper
Player 4 LED flashes 4x - Right Thumbstick
Player 3 LED flashes 1x - X
Player 3 LED flashes 2x - Y
Player 3 LED flashes 3x - B
Player 3 LED flashes 4x - A
Player 4 LED flashes 5x - Sync (for sleeper), or right-tac (for duo)
Player 3 LED flashes 5x - Left-tac (for duo only)
Player 3 LED flashes 1x then player 4 LED flashes 1x - Left Trigger
Player 3, Player 4, Player 3, Player 4 - Right trigger
Player 4 LED flashes 1x - Left thumbstick
Player 4 LED flashes 2x - Left bumper
Player 4 LED flashes 3x - Right bumper
Player 4 LED flashes 4x - Right Thumbstick
Player 3 LED flashes 1x - X
Player 3 LED flashes 2x - Y
Player 3 LED flashes 3x - B
Player 3 LED flashes 4x - A
Player 4 LED flashes 5x - Sync (for sleeper), or right-tac (for duo)
Player 3 LED flashes 5x - Left-tac (for duo only)
Player 3 LED flashes 1x then player 4 LED flashes 1x - Left Trigger
Player 3, Player 4, Player 3, Player 4 - Right trigger