I got 1. e7 Kf5 2. Kf8 Kg6 3. e8=Q+ Kf5 4. Qe6# and 1.e7 Kf5 2. e8=R Kg6 3. Re6 Kf5 4. Rf6# but those are both 4 moves? I agree with computer it's 4
There is actually a mate in 3! Hint: you never move the e6 pawn.
Update! 1. Be8 Kf5 2. Kg7 Ke5 3. Kg6#
That's correct! I assume by Be8, you meant h8