1990 240 auto not shifting to 4th gear

[jfletch]

My 1990 @40 Dl auto trans is not shifting into fourth gear. When i bought the car about a month ago it shifted sometimes and now it rarely ever does, so I dont go on the highway. I checked the overdrive relay and the up arrow goes on and off when i push the button on the shifter and the relay clicks also. i am not sure what to check next. any help would be great.