We're starting with someone who, if that fetching headdress is any indication, is a member of the Selesnya guild.
This next booster also features someone with unusual headgear. Unless those are head-tentacles. Which they probably aren't.
Ah! This would be the new Niv-Mizzet (the Dracogenius).
Ooh. Spooky. Well, I find it spooky, although i guess it's possible that what I take to be a skull-face is really just a white mask. Or even just a white face.
And finally, a soldier of some sort. If we assume that the top booster was Selesnya and the third was Izzet, that would suggest that this one was Azorius. And the crazy-looking second booster would be Rakdos, leaving Golgari for the fourth one. Logic!