4. Ba4 b5

This is what I would consider to be a "semi-common" situation.  Obviously 3 ... a6 is fairly common after a knight to king or queen pin, (Particularly in this example, I'd like to focus on the pin found in the spanish)  But sometimes I will find a player who likes to push the b pawn and force my bishop onto the kingside castled kings diagonal (which with rare exception is where the king will be casteling to).  I almost never mind this, since I really haven't lost much(as far as I can tell) by moving my bishop for 3 consecutive moves since black hasn't been developing and I am now on the kings diagnal.  I'm very curious though, because these are a developing move in the sicilian usually preparing to bring up the queenside bishop...

Really I would just like to hear what some of the more experienced players think about this pawn advancement for both black and white.

It's fine for both, look up Zaitsev, Archangel/archangelsk, and Karpob variations.