Well at first I was wondering if they were supposed to be different advancements of turret, but while I could see 1 being upgraded to 2 and 2 upgrading to 3, 3 to 4 doesn't make any sense at all visually. It doesn't seem like they are different turrets entirely either, since they all have the same guns. Unless of course those guns are just placeholder and any turret can be loaded with any gun type (I wonder if you could do a different weapon on each side like a MAX can do).
The NS thing seems to indicate these are all non empire specific turrets (although that may be an unfair assumption), so aside from the possibility that different bases have different looking turrets, a vote on which one was your favorite seems like the natural progression to me
For me I would say it's 1 > 2 >> 4 >>> 3
1 is my favorite, but 2 is pretty awesome as well. I don't really like 4 that much and I really don't like 3. Just too fat and bulging for my aesthetic taste.
But all four would do and they are all well designed. Even four wouldn't put me off if it weren't for how awesome 1 and 2 were. I've been pretty happy with where their art direction has gone in this game. Aside from some of the NC stuff, it's all looked really great, and even the NC stuff has amazing detail poured into it. Just looks ugly to me though.