search superior shift kit.
and K1 k2 spring kits.
call Roy and ask his advice... he's on the road last I checked, but he left his cell phone # on VM if he is!
it could be a simple vacuum adjustment or modulator adjustment. because usually when you shift into 4th, you have lifted your foot a bit as you have achieved the speed you were hunting, and the vacuum pressure may need to be lessened for that throttle position.