They had both a Mark IVb combo and a new Mark V combo at Guitar Center, and I got to play them side by side. I also plugged them into a Road King cab.
I was prepared to buy the IV and sell the V if I determined the IV was really better. But I didn't hear it, not after over an hour of play. Yeah, the IV allows you to tweak the lead channel a bit more. But whenever I got a sound I liked out of the IV, I could reproduce it almost exactly from the V. In fact, the V sounded a bit better--it was thicker on the high notes than the IV. The IV had a slightly more raw sound than the V, and the V was a bit more compressed. The V actually sounded like it had more available distortion. And the extreme mode on the V was clearly better than on the IV--it sounded more modern and heavier.
Of course, the V dominates the IV on channel 1 and especially channel 2.
So, I'm pretty satisfied that I should stick with the V.