I also have a 1986 300sdl. An absolutely wonderful car!! I had some slippage between 3 and 4th gear also. My slippage occurred when the car was cold, and then slowed down in slippage when the car was at operating temperature.
My culprit was a device called the Temperature and boost pressure dependent vacuum control system. The softness of the shifts in the tranny depend on the vaccum applied to the vaccum modulator on the automatic transmission.

When the engine is cold there is a temp switch that controls a vacuum amplifier switchover valve. This valve SOFTENS the shifting of the transmission when the engine is cold. Softening the shifts can equate to slippage.
When the car reaches 50 degrees celsius in water temperature this opens the ground from that temp sensor switch and now there is a decrease in vacuum going to the modulator at the tranny and the shifts become more firm. hence less slippage.

Experiment: disconnect a vacuum line under the hood of the car and drive the car. Loss of total vacuum and the shift will be very hard and firm, like the shifts are banging. Reconnect the vacuum line and watch the shifts get softer.

I disconnected the electrical plug from my vaccum amplifier switchover valve and now my cars shifts are a little harder and I have no slippage whe.n the car is cold.

John