My doctor says he could only remove 4000cc. He's taking fat from the inner thigh and I'm getting lipo 360 done

If it is total fat after liposuction then that is the amount allowable by law (in Florid anyway), and that usually equates to about 8000 cc of total Lipo aspirate which is a significant amount of fat and  2000 cc into each buttock is a very large transfer and I would say not possible or desirable in the vast majority of patients.

That amount is in line with what most plastic surgeons would remove as an outpatient.  These guidelines are for your safety and may vary slightly from surgeon to surgeon.  

The amount of fat being removed will determine whether or not you can be taken care of as outpatient, and most surgeons use a 4000-5000cc limit for outpatient surgery.  You should speak with your doctor about the reasons for the recommendations.