C4 Problem

Sounds like your linkage adjustment is off a little. You will get a lot of different procedures to adjust it properely and this trans is easy to adjust improperly. What I would try is put your shifter lever in neutral, jack the rear wheels off the ground or put on a lift and put the trans lever in neutral and verify by rotating back wheels and watching drive shaft to confirm its actually in neutral. You can then insert a drill bit thru the neutral safety/reverse switch and thru the shift linkage to line it up. you may have to loosen the switch bolts a little to fine tune.