An oldie but a goodie

Some of you may have seen this one before, but White to play and Mate in 4.
(Composed by Lord Dunsany, although I sourced it from Origami, Eleusis and the Soma Cube by Martin Gardner, which was a very recent purchase. I suspect I may have published a similar puzzle in one of my Xmas quizzes, or even put it up here at some point).