Personally, I would try ignoring it and playing the hard nut role.
If he calls you this name and then you punish him and then he says that if you don't retract the punishment he will call it you again, then he is making the rules. He is only 4 and 4 year olds aren't allowed to make the rules. I appreciate that we teach our children resect by respecting them but you need to respect yourself also. I would try playing the card that he can do what he wants, what do you care if he calls you a b1tch? you will only add to the punishment until it stops. You are not trying to beg him to stop, you are telling him to. You punish him once and stick to your guns no matter what. If he does it again, then add to the punishment again, eg, first offence, no TV; second offence, no TV and no playstation; third offence, no tv, no playstation and no sweets for a week etc etc. he has to understand that he cannot blackmail his way out of it.
Parenting is never easy and the fact that you are upset by this and trying to correct it makes you a wonderful and caring parent. Remember also, that all things come to pass and he is not going to introduce you on his wedding day as "B1tch of the groom". He is perhaps trying to find out how much he can push you and is starting to exert his masculinity over you. I have the exact same thing with my 4 daughters (obviously trying to run my house for me) but no such issues with my son. However, my husband and my son are continually sparring for top dog position. I think it may be a little to do with age and same sex child to parent relationship. It is because your role is more unusual that it is not as talked about as problems wives have with their daughters.
Good luck and I hope it sorts itself out soon. Things like this often do. especially if the reaction becomes undesirable to the perpetrator and also boring.