I've had a few Marks over the years...V25, V90 (twice), IVa, IVb, JP, and a VII. My favorite probably was my IVb. I had the JP and VII at the same time. The VII is like the most modern sounding Mark. More gain, tighter, more aggressive. The JP feature set is the best. 3 channels, 2 EQs, and Shred mode, but I thought the VII sounded better than the JP. The JP has this slight boxiness going on that you can't really dial out. The VII is more open.
The IV has a bigger low end and the overall tone is a bit low mid focused compared to the others. I think that is a unique character to the IV that makes it my favorite.
All that said, they all sound great, and you really can't go wrong. I was happy playing each one of those amps while I had them. But of all of them, I wish I kept my IVb.
I wish they did end up doing a Mark VI with the clean, IIC+, and IV channels set up like the JP, with 2 EQs and Shred built in. To me, that would be the perfect Mark series. I liked Extreme mode on the V a lot...shared some midrange character with a 5150, but overall the V needed more low end.