A funny move, but if we consider that Carlsen has played 4.a4 here, this one is not so exotic. I imagine the idea is to see how Black will develop. If he plays 4...d5, then the white bishop soon goes to b5, and Black doesn't have the option of Bg4. If 4...Bc5, then White can play 5.Nxe5. 4...Be7 is too passive.
1. e2e4  1... e7e5  2. g1f3  2... b8c6  3. b1c3  3... g8f6 