On move 4 you should have played Qe2 Nd6 Nc6+ Black can resign. My chess instructer told me about this position but there is conter-play with white slightly better.
The opening black played is the Petrov's Defence but black played it incorrectly. When 3...Nxe4 you immediatly play Qe2 and if he runs his knight away to like f6 or d6 you simply play Nc6+ winning blacks queen. The correct opening line for black is to play 1.e4 1..e5 2.nf3 2....nf6 3.Nxe5 3...d6
Here is a game I played against my electronic chess game.