I have a 92 Ford mustang w/4cyl engine & auto transmission. The overdrive...

Those Mustang trans gave a lot of trouble. My first impression is that overdrive has come out, but some of those models actually had an electric switch that triggered the overdrive. Why don't you go and get a repair manual at Advance Auto and see if it has naything about a trans relay. I am not sure if that model had a solenoid on the trans or not. If it does it may be internal and will need to be done by a trans shop. Good luck.

No experiences, but with the age maybe the ECU has lost that demand to go to overdrive. Or it could be that the 4th gear isn't going when told from a bind.