Chess Opening Theory/1. e4/1...c5/2. c3/2...Nf6/3. e5/3...Nd5/4. d4

4.d4
This move gains central space and connects with the e-pawn. White has created a pawn chain in the center and is now threatening c4 followed by d5, when Black would really suffocate by his lack of central space. This pawn controls White's center.