i think this is what you meant as far as equivalent widths:
compact combo=short head (rackmountable), 18 3/4 in
widebody combo=medium head (not mountable) 22 1/2 in
well, if you can find someone who you're willing to pay big $$ to, you could probably convince them to make a non-standard rack for you for mounting the mark iv head in. but you're going to have an easier (and cheaper) time finding someone who's willing to trade a short head for a widebody combo.
one other thing you can tell is that the widebody combo/medium heads have the graphic eq and lead voicing switches on the front panel, just to the right of the eq (see pic on mesa site). http://www.mesaboogie.com/Product_Info/Out_of%20_Production/Mark_IV/Mark_IV_Closeup/mark_iv_Panel_closeup.html
short heads will have these on the rear panel.
hope that helps.