momentary switches to control shifting

haha, I told you how I would do it, I would use a microcontroller.  The amount of time it would take to design the complete circuit without using a microcontroller is unfortunately more time then I have right now. 

However, based on your criteria, it really shouldn't be too hard, since you arn't looking for two button control (downshift and upshift).  Using 4 buttons should greaty simplify the operation of the entire circuit.  I suggest starting with 4 latching relays and the four buttons and work from there.  The trick will be getting the currently selected relay to unlatch when another button is pushed, and getting the module to start up in the right gear (so you don't blow your tranny up).