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

4.e3
Considered to be the best move, this frees the b4-square for the c3-knight and prepares to round up c4 with Bxc4. Black should prevent this with 4...b5. This also sets a deep trap for white. This leads to very complicated variations on the queenside.