4...Bg4 5.Be2

The idea

After 4...Bg4 5.Be2, Black's bishop pins the knight that supports e5, and White unpins with the bishop. Wikipedia calls 4...Bg4 the most common response and 5.Be2 White's usual reply. Lichess calls 4...Bg4 the Modern Variation's main line.

Plans for White

Castle, play c4 to chase the knight, and keep the center. If Black takes on f3, recapture with the bishop to keep the pawns intact and get pressure on the long diagonal.

Plans for Black

Continue with ...e6, ...Be7 and ...O-O, and decide when to trade on f3. Black often gives up the bishop pair with ...Bxf3 to weaken White's control of e5 and d4.

Watch out for

For Black, trading too early. ...Bxf3 gives White the bishop pair, so it should come when it wins time or damages White's center.

For White, the pressure on e5 and d4. With the knight pinned or traded, both pawns have fewer defenders, and ...Nc6 can hit them.