Re: 4 vintage 30's or 2 greenbacks+2 vintage 30's ?
I like the idea of mixing speakers and think its good to do but i wouldnt go with vintage 30s and greenbacks. I personally am not the biggest fan of greenbacks, although all im going on is what it sounded like through a new marshall AX cab and a DSL and i really dont like new marshall stuff, but i did have my ENGL vintage 30 cab in the same room at the time and the vintage 30s sounded heaps better in my opinion. Much tighter but still with that soggyness about them that i love.
I think with whatever you get you can just sell them and swap them as many times as you like anyway. Gear's combo of g75s and v30s sounds like a good combo, but id also give the g12h30's a go, they sound like an awesome speaker although ive never heard them in person.
Id go with 2 vintage 30's and 2 g12h30's, great for rock and blues alike i would think.