After the initial moves 1.d4 Nf6 2.c4 c5 3.d5 e6, the moves 4.Nf3 and 4.g3 are sometimes played, but White's most popular move is 4.Nc3, preparing to support his d-pawn with 5.e4.
After the initial moves 1.d4 Nf6 2.c4 c5 3.d5 e6, the moves 4.Nf3 and 4.g3 are sometimes played, but the positions arising after those moves are more commonly reached via transpositions. Instead White's most popular move is 4.Nc3, preparing to support his d-pawn with 5.e4.