4. ..Bxc3+ is not necessary. I think you should wait until white has played a3 - now you are basically giving him an extra tempo for free.
I think your 'trap' is best answered by 7. Nh3 followed by f3, Nf2, e4, Bd3 etc.
Interesting trap in the classical Nimzo Indian


Listen, son, you could learn a lot by studying the Master.
The National Master?
I found this entirely by accident in a recent game: