Chess Opening Theory/1. e4/1...e5/2. Nc3/2...Nf6/3. f4/3...d5/4. fxe5

=Vienna Game= As Black knight is under attack, the natural move is /4...Nxe4/.

Theory table
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1. e4 e5 2. Nc3 Nf6 3. f4 d5 4. fxe4 Nxe4