Firebatx

Is it possible to create a shortcut on your keyboard that triggers another key.

Basically i want to be able to press MOUSE4 and MOUSE3 is triggerd.

But.....

I only want it to work at certain times, i want to be able to use MOUSE4 as default,

I only want it to change when i am in a specific Aplication, i can bind MOUSE4 to MOUSE3 in that but i cant get MOUSE3 to do its specific job when that is done, as MOUSE3 is being used by a diffrent program in windows. Is there a way of creating a script or .exe that when i press MOUSE4 in the aplication it will load MOUSE3 within windows, but then when im not in the App it wont?

Please Help Thanks!