Chess Opening Theory/1. e4/1...d6/2. d4/2...Nf6/3. Nc3/3...g6/4. f4/4...Bg7

=Austrian Attack=

4...Bg7
This is Black's normal continuation. Black continues to aim his pieces at the growing White center. White's center poses a growing threat to Black's position. The advance of the e-pawn for White would snuff out the bishop's activity, so Black must find some way to prevent e5. He is safe for now for tactical reasons, but preparation by White will force Black to take preventive measures.

Theory table
1.e4 d6 2.d4 Nf6 3.Nc3 g6 4.f4 Bg7