Chess Opening Theory/1. e4/1...e5/2. f4/2...exf4/3. Nf3/3...d6/4. d4

d4 opens up the bishop to recapture the pawn, and grabs more central space. Black usually responds with g5, temporarily defending the pawn while planning ...g4 to trap the f3 knight. Believe me when there is a line where white sacrifices the knight on f3. :)

Compared to 4.Bc4, white usually castles QUEENSIDE with this variation, since white develops their queenside pieces and moves their queen usually.