For the Force RS, both the throttle and brake should be set on the Y axis - when you calibrate the pedals in GP4, they will each take half the Y axis range.
Make sure that in the Properties dialog for the ACT-Labs Force RS controller in the Games Controllers option in the Windows Control Panel, the throttle pedal moves the + up and the brake pedal moves the + down (i.e. both on the Y axis). The checkbox ('Use pedals separately'?) should be cleared.
I find the Force RS wheel works very well with GP4, although I have yet to find the best setting for the force-feedback - GP4 doesn't really do it justice...
Dave