Black still lacks space. He must at all cost avoid a passive play which would allow White to crush him quickly.

Three options are playable :

4...Bg4 reduces White's control over the center. Black often accepts to break his bishop pair and takes the knight.

4...g6 prepares king's bishop fianchetto.

4...dxe5 is sometimes seen. It frees space for Black but leaves him with no weapons to counter White's advance in development. Moreover, this move seems to be out of the "spirit" of the opening.

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'1.e4 Nf6 2.e5 Nd5 3.d4 d6 4.Nf3'

Wikipedia has related information at Alekhine's Defence.

• Kasparov, Garry, & Keene, Raymond 1989 Batsford chess openings 2. ISBN 0-8050-3409-9.