Continuing from the solution described by Friedel and Troitzy,

 1.Bc6!! Rb1+ 2. Ke2 Rb2+ 3.Kd3 Rb3+ 4.Kc4  

Black can also play 4 ..Rb2 (to avoid mate on g2, instead of 4.. Rf3 as
given in the solution). Then the winning continuation is -- 5. Qc1 
(threatening 6. Qh6 mate or winning the rook).  

If Black plays 6. .. Rb4+ then, 7. Kxb4 f4 3. Qf1+ g2 4. Qxg2# 
If Black plays 6. .. Rc2+ then, 7. Qxc2 (and mate on g2 is unavoidable next move)