Assuming Avatarspeakers aren't available to UK, I'll go ahead and recommend speakers.
Well i use classic lead 80's with my mark IV. They are very well balanced, darker sounding than V30's. The good thing about that is, since mark IV is a crazy flexible amp, you can still sound bright out of cl80's. But since you can sound dark if you want, it won't be a problem ieither. So you cant good flexibility. Trust me. WHoever came into my house to check amps with the Avatar i have, they liked it alot! As for 4x12 or 2x12, I would go 4x12 but I honestly didnt wanna spend that much and wouldn't like to carry around a 4x12. Pretty happy with my 2x12 though. I also have a 4x12 Marshall cab and Avatar is actually sounding punchier, thicker and stronger!