[s_nova69]

Having the TV cable set correctly is very important. The TV cable regulates how much hydraulic pressure the clutches see in a specific gear. An improperly set TV cable will not create a pulse in 4th.
The converter locking up in 4th would not really feel like a pulse. It would feel more like a shift.

I would look more toward your carburetor, fuel pressure, timing, timing advance mechanism.

Like mentioned before I would first turn up the advance to 10-12 degrees advance. A vacuum gauge will also help to see if that is the problem. If the vacuum gauge moves a bunch when it is "pulsing" you will know there is a problem when under load. If the gauge does pulse then it could be timing or fuel pressure or carb tuning problem.

I wonder if it would do it under the same type of load in another gear.

Just my two cents