4+0. Colour the pieces. (a) diagram; (b) Rg6 -> g7
(a): because both Kings can never be in check simultaneously, one of them is in double check by Rg6 and Qh8. The only possible explanation to this double check is that the last move was -1. g7xh8=Q+. Thus we conclude that Kh6, Qh8 and Rg6 are White; Kg8 is Black. (Last move is not completely determined because both g7xBh8=Q and g7xNh8=Q are possible.)
(b): here the Q gives check to one of the Ks, without any apparent last move. In fact, last move was -1.h7-h8=Q+, enforcing Kg8, Qh8 and Rg7 White; Kh6 Black.