1989 Mercedes Benz Transmission Not Wanting to Shift into 4t

Transmission problem
1989 Mercedes Benz 220000 miles

Hello Carpros, First thanks for the site - it is very helpful. On to the question, my 1989 300 MB does not want to shift into 4th gear after the car warms up. When the engine is first started and cold, the transmission performs great. It does not slip, even when warm. After the engine warms up, gears 1 thru 3 fine, but will not shift into 4th. Any ideas because I am lost. Thanks a Million.