94 E320 no fourth gear

[beerdavis]

I have a 94 E320 wagon. I have 80k on a rebuilt transmission. The transmission goes into reverse and shifts well for gears 1-3, but will not go into 4th. I have been driving the car with the shifter at the 3 position around town and the car drives great. Transmission fluid and filter changes every 30k The fluid level is correct and looked clean. I went ahead and changed the fluid and filter and there was no metal in the pan. any ideas where I should look next. I don't want to put another transmission in there so soon.