A doctor recommended two sessions of GPS lipo one week apart. Is that safe? I have over 4 liters of fat to remove.

This is very difficult to evaluate without pictures or an exam. Many
varieties of liposuction are available including ultrasonic, smart lipo,
laser lipo, vaser lipo, power assisted lipo, and conventional tumescent
lipo.  The results depend very little on the method used and very much
on the surgeon selected. 
The most common postoperative issue of liposuction is contour irregularity. Because contour irregularities may be secondary to postoperative swelling and skin elasticity, they may be treated conservatively for at least 6 months after the initial operation.The only limit when it comes to fat removal via liposuction is the total amount that surgeons are allowed to remove during a single surgical case (5L).

The usual safe limit is 5 liters, which corresponds to 10 lbs of fat at one time.  Unfortunately, you have provided photographs or told us your height & weight.  Certainly, 4 liters can be done in 1 session.Best Wishes,Gary Horndeski, M.D.

The choice of the having one or two liposuction/BodyTite procedures depends on many things including the locations of fat (one body area vs. many body areas), type of anesthetic used (tumescent vs. general anesthesia), health of the patient, the BMI of the patient and son on. It generally does not depend on the doctor board certification (dermatologist or plastic surgeon). Some procedures remove just fat and others can remove fat and tighten the skin.
BodyTite removes the fat and tightens the skin. Combination of exercise, healthy diet, decreasing weight and BodyTite is the best. At the same time, it is important that all prospective patients get in-person detailed examination.I have been performing BodyTite Liposuction for over 7 years and it is a great technique for fat removal as well as skin tightening.Please make sure that you see an experienced doctor for a thorough consultation and assessment prior to undergoing any treatments.Best regards and I hope this helps, Dr. Sapijaszko (Fellow, American Academy of Cosmetic Surgery)