Addition to Alena’s analysis below:

1. Rc5!! bxc5, 2. Nxb8+! (discovered check) c6, 3. Na6 cxd4, 4. Nc5#

I was solving this puzzle during one hour and I managed to get mate in 6. After that I decided to look at the position again. Suddenly I saw that I had forgotten to put the white rook on d5. As soon as I put it in place I solved this puzzle. Actually it was very easy.

V-1

1.Rc5! Bb1
2.Na5+ c6
3.Nb3 Rh8
4.Nd2#

V-2

1.Rc5 Ba7
2.Nb4+ c6
3.Na2 Rh8
4.Nc3#

Rc5 is a key move because it protects the king from check.