According to this page, Y would be Button 4.
Stick 1 = left analog stick
Stick 2 = right analog stick
POV = dpad
Button 1 = A
Button 2 = B
Button 3 = X
Button 4 = Y
Button 5 = LB
Button 6 = RB
Button 7 = back
Button 8 = start
Button 9 = left analog stick center pushed in
Button 10 = right analog stick center pushed in
Button 11 = left trigger
Button 12 = right trigger
Button 13 = X silver guide button
