400cc of fat is an estimate. The only time the surgeon would know the volume of fat available to him is AFTER the fat has been suctioned and separated from fluid, blood and dead fat.
It IS a very small amount considering that the range of fat transferred varies from 350cc to over 1,000cc PER butt cheek. But, the result would also depend on your size, your butt size and your desired buttock size. If you are petite and would be comfortable with an obvious rounding and fullness which are not "out of this world:, this may work. But if you are a larger athletic woman with flat butt cheeks who wants a really full round look, 400cc would not be sufficient in my opinion.