With my Mark IV, I've used various combinations of the following:
1) Celestion Custom 90 in the combo
2) Celestion Custom 90 in a widebody, 3/4 back cab
3) EV 200w in a Thiele ported cab
4) 2
Celestion Vintage 30's in a closed back cab
5)
Genz Benz G-Flex (2 GB speakers in a ported cab)
So far, my favorites have been the EV/Thiele cab and the G-Flex cab--both of them provide 2 things I find the Mark IV needs a little help on--punch & low-end clarity. Every time I've tried to run the Mark IV through Celestion speakers, the end result feels just a little loose, borderline muddy. On the other hand, I think the Custom 90 has a certain warmth that the EV doesn't have. The best of both worlds, perhaps, is the Custom 90 in one cab with the EV in the other (that's the setup I've got on my backup rig now).
--B