1.R*g5 ? zugzwang

 1...Kh3 2.Sf6 threat: 3.Rh5 + D Kg3 4.Se4 # B ( model mate )

 2...Kh2 3.Se4 zz B Kh3 4.Rh5 # D ( model mate )

 but 1...Kh4 !

1.Ra1 ! threat: 2.Sf6 A g4 3.Se4 + B Kh2 4.Rh1 # C ( model mate )

 3...Kh3 4.Rh1 # ( model mate )

 3...Kh4 4.Rh1 #

 1...Kh4 2.Rh1 + C Kg3 3.Sf6 A threat: 4.Se4 # B ( model mate )

 3… Kf2 4.Se4 # B ( model mate )