Itchy rash 4 weeks after BR: This rash popped up a week ago. (Photos)

This is often related to the garment/bra and using some mild steroid cream may help. I would ask your PS for advice and if not resolving see a dermatologist for advice.

This does look like a fungal rash to me.  I would recommend talking to your plastic surgeon about a Nystatin (anti-fungal) powder rather than a cream.  It is very important to keep the area clean and dry.  You may also need a bra that allows room for the incisions to breathe.