replace one of gamepad buttons with number 4

Hello
I have an android box and use it to read pdfs that are converted to jpg format.
I have several problems with the photo part of kodi but one of them is the most annoying.
When I want to zoom, I use up and down bottons.
But it doesn’t zoom step by step and either zooms all the way in or all the way out and I have to press them multiple times to get it almost right.
when I connect a keyboard it appears that I only need to press number 4 to get it right.
I was wondering wether it is possible to replace one of gamepad trigger buttons to number 4.
How can I do that? is that even possible? 
Thank you