1. d4 Nf62. c4 e63.Nc3 Bb44. Qc2...4. Qc2 is the mainline, but why not Qc2
4. Qb3 is dubious since it allows following continuation: 4... c5 5. dxc5 Nc6 6. Nf3 Ne4. Notice that after Qc2 it is impossible for black to play Ne4, since queen covers e4.
Thanks, that helped a lot!