Does your volt meter twitch or drop off when it shifts into fourth? I'm having a bit of brain lock here and really can't think of much that would cause issues, just in fourth gear. All I got is some maybe's.
Low vacuum on a cylinder or two that is causing a lean condition, that cases bucking. Tends to be more noticeable when you are cruising at a constant RPM and may be hardly noticeable as you accelerate through the lower gears.
Ignition usually causes a poor idle and/or breaks down at higher RPM's.
Try riding the brake just a little, just enough to turn the stop lights on. This will disconnect the torque converter lock up and take it out of the equation as a possible issue.
I guess it is possible a tranny shift solenoid has low resistance and is sucking the voltage down to other motor management components. And/or you have a voltage issue.
The TPS is two parts (two separate TPS made as one unit with the automatic transmission) on the Renix, the transmission half and the ECU (engine) half. The engine half has a lot to do with spark advance and may be your issue. I've seen a few failure modes in the TPS, first is poor ground, second is it is full of water, solvent or oil, the third is it has a dead spot, fourth is the connector pins for the TPS are corroded, covered in oil or coolant. In my experience if it is going to act up, it wouldn't be just in fourth gear, but maybe it is a very specific chain of events that makes it most noticeable in fourth gear?
I'm trying to get a sense of whether it is a tranny issue or a motor issue, this can sometimes fool you.