Mark IV heads and combos are pretty much the same thing. The only real difference is the size and weight, and perhaps subtle nuances in tone due to speaker and tube selection. The combo is obviously going to be a bit bigger and heavier because there is a larger shell enclosing the chassis and a twelve inch speaker. The head is just the chassis enclosed in a smaller head shell. The amplifiers themselves are going to the the same, remember, with Mesa you can move a chassis from a combo to a head, or vice versa. The only sound differences are going to be due to speaker selection, tube selection, and whether it is an A or B version.