Pseudo-Austrian Attack

The idea

After 4.f4 c6 5.Nf3 Bg4, White has a broad pawn center and Black pins the knight that supports it. Wikipedia gives this as the Modern's standard continuation. It's called the Pseudo-Austrian because Black hasn't played ...Nf6, unlike the Pirc's Austrian Attack.

Plans for White

Play Be3 and Qd2 or Be2, castle, and look for e5 to gain space. h3 can put the question to the bishop. White's center is the main asset, and pushing it forward can cramp Black.

Plans for Black

Trade on f3 to weaken White's control of d4 and e5, then hit the center with ...Qb6, ...d5 or ...e5. Black's pieces are aimed at d4, and the pin makes it harder for White to defend it.

Watch out for

For Black, the e5 push. With f4 played, White can push e5 quickly and gain space, so Black's counterplay must come first.

For White, d4. After ...Bxf3 and ...Qb6, d4 and b2 can be hit together, and the bishop on g7 adds to the pressure.