Chess Opening Theory/1. d4/1...d5/2. c4/2...c6/3. Nc3/3...dxc4/4. e3/4...b5

4...b5
This guards c4 and is the best move. The queenside weaknesses are not a problem, as the knight on c3 is vulnerable to b-pawn attacks with ...b4. Note that Nxb5?, trying to exploit the long diagonal to destroy Black's queenside, is a mistake, and gives black two roads to an advantage. Often, Black will play b4, to boot white's knight away, and not try to hold c4.

When your opponent has a spatial plus, you want to use pawn breaks. Since b3 Qa5 (b4 Na4 c3 is also good) is annoying, the only other break is 5.a4. Therefore, the best move is 5.a4.