Would you let your sons stepmother get away with this?

Personally I would just leave it. Had she put mother of three then you may easily have felt upset that she had not included your son. She has likely put mother of 4 for ease and to avoid confusion and even to avoid singling any of the children out. Your son is clearly part of their family and she is now married to your ex-husband. Have a moan to a few of your friends if necessary but otherwise I would let it go....life is too short :)