4.Bd3 with ...Bb4+ and ...f5

The idea

After 4.Bd3 Bb4+ 5.Nc3 f5 6.Qe2 Nf6, Black has attacked e4 three times, with the bishop on b7, the pawn on f5 and the knight on f6. White holds it with the bishop on d3, the knight on c3 and the queen on e2. This is a line Wikipedia gives as a main one.

Plans for White

Keep e4 protected and finish development with Nf3, Bd2 and castling. If Black takes on e4, White recaptures with a piece and uses the open lines. White can also play f3 to support e4 for good.

Plans for Black

Take on e4 when it wins material, or keep up the pressure with ...O-O and ...Nc6. The bishop on b4 pins the knight that defends e4, so trading it with ...Bxc3 at the right moment can win the pawn.

Watch out for

For White, the pin. The knight on c3 can't defend e4 freely while the bishop on b4 pins it to the king, so watch the count of attackers and defenders on e4.

For Black, the e6 pawn and the light squares. With ...f5 played, e6 is weak, and if the e4 pawn opens up, White's queen on e2 hits it down the e-file.