no probalo!
the mark III has an awesome tone to it, but theres almost no compression in it anywhere. the mark IV is more compressed (not as much as say, the rectos, but still pretty compressed) so things like gain, presence, volume controls, and pick attack are a lot more forgiving. with the mark III you really need to play the amp to get the sound you want. I use R1 as both my clean and crunch tones, R2 (once I get the R2 mod for volume control) will be my main rhythm tone, and LD will be well, lead. a lot of times with my settings anyways, the difference between not enough gain, and plenty of gain is how hard I pick. and this applies even to the lead channel. as far as the IV goes, I actually prefer the lead of the III to the IV. the III is much less compressed, and less buzzy. if you have a fizzy mark III, turn down the presence and it can go away. the IV is much more likely to be fizzy/buzzy. also the lead of the III is less compressed, so it comes across punchier, bolder, and much more raw. when I play with my band, its basically straight raw hard rock. so I like the raw boldness of the III