Car does not shift into 4th on 1991 Volvo 740

The overdrive engage/disengage on the left side of the shifter sometimes works and sometimes it doesn't. The up-arrow light is on. It will go off intermittently and when it does, the RPM's are definitely lower, telling me that it will go into 4th. Am I looking at an overdrive relay problem or am I looking at an overdrive solenoid? How hard is the fix and what kind of money can I expect to pay.

Asked by for the 1991 Volvo 740
There's a good chance this is the overdrive relay. Not sure how much the part is, you can find it at the dealer or online. A person with a phillips screwdriver and a little mechanical ability can replace it in 5 minutes.
The relay is under the ashtray in the center console, looks like this: