4.Nf3 Bb4

English Defense

1.d4 b6 2.c4 Bb7 3.Nc3 e6 4.Nf3 Bb4 5.Qc2 Nf6

Black pins the knight, and the game heads toward a Nimzo-Indian kind of position.

The idea

After 4.Nf3 Bb4 5.Qc2 Nf6, the position resembles the Nimzo-Indian Defense, as Wikipedia notes: Black pins the knight on c3 and controls e4 with the bishop on b7 and the knight on f6. White's queen on c2 also guards e4.

Plans for White

Break the pin with a3, and if Black trades on c3, recapture with the queen to keep the pawns intact. Then play e4 when it's safe, or g3 and Bg2 to fight for the long diagonal.

Plans for Black

Decide whether to trade on c3. Keeping the bishop and playing ...O-O, ...d6 and ...c5 is solid; trading gives White the bishop pair but stops e4. Black's control of e4 is the key factor.

Watch out for

For Black, losing control of e4. If White gets e4 in with support, Black's pieces lose their main target.

For White, the pin. Until it's broken with a3 or Bd2, the knight on c3 can't move without losing material, and ...Ne4 can add pressure to it.