Firstly, as black I would play 4.cxd4 rather than Bg7
Secondly, as white I would play 5.d5 rather than dxc5.
Also, I think it is ok for black to play Bxc3 followed by Qa5, although you should aim to get a good dark square grip in the centre with your pawns.
This is a trap in the English opening. I has nothing to do with mate, but black can sacrifice a pawn to watch white have weak tripled C pawns
Please comment and tell me if that is sound for black to do
Please