Chess Opening Theory/1. f4/1...e5/2. fxe5/2...d6/3. exd6/3...Bxd6/4. h4/4...Bg3

4...Bg3#
This unexpected move checkmates the white king along the e1-h4 diagonal in a resounding fashion.

'''1.f4 e5 2.fxe5 d6 3.exd6 Bxd6 4.h4?? Bg3#'''