1994 e320 wagon tranmission erratically hesitates shifting into 4th gear under normal driving conditions. Shifts 1st thru 3rd no problem experienced. What controls the shift from 3rd to 4th? Would any related codes be present at the diagnostic socket, which I should look for? Any recommended past post which address this type of shifting pattern?

You have to start with the basics like modulator pressure and bowden cable with vacuum check.

Next, the clutch that applies 3-4 is located in the very bottom of the tranny. You will have to do a complete disassemble to check it.