4...Nc6 is the usual continuation. the game explorer, if you don't know yet, can give you the alternative moves.

After 1. d4 Nf6 2. c4 e5 3. dxe5 Ng4 4. Bf4 white doesn't have a best move because it is black to move. I believe Black's most popular moves are 4. ... Nc6 and 4. ... Bb4+.