If you want in general more cut in the band mix and more aggressive mids, the Eminence Governor (or, more expensive but almost identical sounding Celestion V30) will help with that. I think two speaker would be enough, just make sure they match the impedance of the other two speakers.
If you use super low tunings, two Eminence Black Powders are very tight. If you want more volume and very good and versatile sound, the Eminence Wizard (or, again similar, Celestion H30) will help with that.
Swamp Thangs have also been recommended very often on this forum.
If you want to spend some more money, the Eminence Delta Pro 12 is a cheap but good Electro Voice EVML12 clone. I think that will also help with very low tunings and low end, but be more balanced than the Black Powders.
For fat sounding blues and clean jazz runs, I love the Eminence Cannabis Rex. it is very fat and rounded and has very low break up.