Yeah, I don't understand why there is no expression to get gamepad number.
I guess for events like "On key pressed" you'll have to create 4 identical events for each gamepad number. Then redirect them to a function, like this:
On Gamepad 0 button A pressed -> Call Function ("Jump", 0)
On Gamepad 1 button A pressed -> Call Function ("Jump", 1)
For events like "Is button down" you can test all gamepads in a loop.
By the way, "On gamepad .." events are not very reliable, they can miss buttons if several buttons are pressed simultaneously. Some time ago I made a little engine that only uses "Is.." events, see this post:
It can be modified to support multiple gamepads.