How to respond to 4.b3 in the Benko Gambit.

I've been playing the Benko Gambit in blitz lately and have come across white players playing this seemingly decent move in the Benko. Nonetheless it's not a line played by professionals and I've been curious why.

I always want to "punish" white for playing it, but don't know how, exactly. Is the idea to push the b pawn? That's what I've been doing.

You will run into that move from time to time. I face it about 1 game in 100. 4...e6 is a good reply, immediately going after the center that he's just neglected. 4...g6 is also fine although you always have to be alert that White doesn't suddenly play cxb5 and move order you into a line of the Benko accepted where b3 is advantageous. 4...b4, while a bit committal for my liking should also ensure you a reasonable game.

Interesting. 4...e6 might be a way to go for me.