There are two Mark IV models that practically look identical. Mark IVA and the 1993 Mark IVB, before the wide body change. I have seen other videos, they are calling the IVB a IVA and vise versa. the IVB wide body had the to rocker switches next to the GEQ. The top switch was for selecting the GEQ mode and the switch below it was for lead voicing Harmonics/mid gain. The Mark IVA had those on the back side. I would assume the transition model of the Mark IVB also had the same layout as the IVA due to the smaller chassis. I am not a Mesa Boogie Historian so what other differences there are, I have no clue.
Comparing the two schematics, they almost look similar.
Power amp section does have some differences. The IVB has a relay that changes a coupling capacitor on the phase inverter outputs from 180pF to 250pF (CH3), the IVA has only the 250pF cap.
FX loop circuit is different. The IVB had some sort of mono/stereo thing going on so you can bring the stereo effect back to the IVB and then send it out to the satellite amp. For use with the Satellite 60 or the Simul-Satellite. Never got into that aspect of the amp. (I took this from the manual from Mesa's website). I had the Mark IVB but never made use of the stereo FX loop as I did not have a powered satellite amp to go with it.
The tubes used for the lead drive circuit are different positions. Mark IVA it is V3A and V3B where as the Mark IVB separates the lead drive circuit between two tubes. V3A and V4A along with a control grid to cathode capacitor of 250pf. The Mark IVA does not have that cap in its circuit.
Reverb is managed with V3 on the mark IVA but split using V3 and V4 on the Mark IVB.
The pages for the power supply for the Mark IVA are missing. Hard to tell what if any differences are there.
There are probably other differences as well. Not sure what they are as I did not own a Mark IVA.
Since I am sure that the RHY1 and RHY2 channels retained the shared controls, it stands to reason the model was just improved and did not merit a model change. It was more or less a running change but more dramatic make-over than a running change that is transparent to the customer base.
I guess there were different chassis lengths for the various power sections used with the SRG, DRG, HRG, and KRG configurations of the early marks (IIC and III).
That is almost like having the Mark V90 and all of a sudden there is a Mark V that is compact in size and has some changes to its features, may even have less channels and run on EL84 power tubes. Can you really call that a Mark V? Oh, wait they use the same labels on the amp. Then again, some people got confused about the Electra Dyne and Royal Atlantic, it has similar design cues, channels were labeled the same Lo gain, Hi gain and clean. So, they assumed they were designed the same. Well the preamps were completely different in all respects as were the power sections. The only thing they had in common was Mesa Engineering logo and possibly the preamp tubes.