#1
Okay, I'm buying a head (Ashdown ABM), that has a minimum impedance of 4ohms. I assume that this mean that I should buy a 4ohm cab too? What if I want to add another cab to my rig in the future?

I'm sorry I'm asking such a question, it's just I'm used to combo's, and I didn't know who else to consult.
Ibanez SR505
Ashdown ABM 300 EVO II
Epifani UL410
#2
Buy an 8 ohm cab now (you won't get as much volume from it as you would a 4 ohm) and then if you want to buy another cab later you can buy another 8 ohm and you'll have four ohms. It depends on how much you intend to buy another cab, if you're buying an 8*10 right now you might as well get a 4 ohm because you're not going to very likely buy another cab to go with it, if you're just buying a 1*15 you better buy an 8 ohm because you're gonna want another cab to go with that after owning it a little while.
#3
Okay thanks a lot, I googled it as well, so I got my answer.

Thanks, Chris.
Ibanez SR505
Ashdown ABM 300 EVO II
Epifani UL410