RPM to speed relationship sounds normal for 4th gear in a 6cyl S-class.
How do you know the torque converter is locking? MB transmissions in those years didn't have a locking torque converter?
One way to tell if you have limited slip, lift both rear wheels and spin by hand one wheel with the transmission in Park and park brake disengaged. The wheel should spin freely and the wheel on the other side should spin the other way. If there's resistance, you probably have limited slip. You can pull the rear cover of the differential to know for sure. I doubt your car has limited slip.
I've never heard of 4Matic being offered with the M110 engine or in the W126 body style. If there aren't any drive axles going to the front wheels, you don't have 4Matic.