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

=Giuoco Piano=

4. c3
White reinforces the d4 square, and prepares d4. It also stops any Nd4 by black, in the future.

White also prepares b4 to try and trap the Black bishop.

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