If the wiring is ok, the 4th gear switch may be sticking. The one on my '83 would sometimes work if I held the shift lever to the right and maybe wiggled it a little. When I let go of the lever, the OD would drop out.

The 4th gear switch is on top of the tranny, right side. It is possible to replace it without lowering the tranny. I used one of those deep sockets made for replacing an O2 sensor. This socket has flats on the end which will take an open end wrench. Just clean away the dirt, take off the wire, put on the socket, use a wrench which fits the flats, and unscrew the switch. Takes maybe 20 minutes.