Chess Opening Theory/1. e4/1...e5/2. Nf3/2...Nc6/3. Bc4/3...Bc5/4. c3/4...Nf6

= Giuoco Piano =

4... Nf6
The most common reply. It attacks the pawn on e4, develops the knight, and allows black to castle later on.

Theory table
'''1. e4 e5 2. Nf3 Nc6 3. Bc4 Bc5 4. c3 Nf6'''