I am hardly a guru, but I did some study when we lost 4th on our AW71.

I think the chances are pretty good that the problem is with the OD relay, wiring, or solenoid. Check the 700/900 FAQ for more.

Another external possibility is that the kick-down cable could be too tight or binding, so the transmission thinks you are still applying throttle when in fact you have let up. Check at the throttle drum that the cable is drawn all the way back to the stop clip at zero throttle.

To proove that the problem is internal, you would have to remove the solenoid (tricky) and replace it with some dummy fluid shunt arrangement and go for a drive. Passage "open" is 4th allowed, so capping off won't do.

Theory: Everthing is managed by a hydraulic analog computer known as the "valve body". The transmission will up-shift when the "speed" pressure from the output shaft driven governor pump exceeds the "control" pressure from the kickdown end of things.

Failure to shift to 4th could be due to:

1. too much "control" pressure --- kickdown cable trouble
2. not enough "speed" pressure --- leaking governor pump internal seals
3. 4th logically disallowed --- standard OD issues in FAQ
4. Valve body troubles --- can be overhauled w/o pulling tranny

I think that, in a RWD Volvo, even item 2 can be addressed without pulling the transmission. I would eliminate the the OD control and kickdown cable issues before agreeing to internal work. There should be issues other that failure to shift to 4th if the trouble is internal.

Good luck. These diagnoses are hard, and going into a good transmission is expensive and counter-productive.

Brian