I don't think that there are any really good studies to show when it is safest. You definitely can have your second surgery in 4 days. However, I generally do not recommend this for elective surgery. It is generally not recommended to have prolonged periods of immobility after undergoing surgery due to the risk of blood clots. A second surgery definitely fits into the category of prolonged immobility.
I usually recommend 6 weeks to 3 months, depending on how much was done during the first surgery. Your body needs several weeks to complete the healing process from the first surgery and to regenerate blood cells and proteins and healing factors so that it is ready to heal after the next surgery.
As far as transferring fat from the breast to your buttocks, I would be VERY cautious!!! There is no way to know how many breast glandular cells are being transferred along with the fat. Therefore unless you are planning on getting Mammograms of your buttocks, it could be a very risky thing to do.