4 weeks post BBL. Will I lose more volume?

In most cases there is still some swelling that is evident in the buttock region following Brazilian buttock lift that has not completely resolved. One would expect the buttocks to get a bit smaller. Whether this is a noticeable amount is hard to answer. Usually one should wait 6 to 8 weeks before having a reasonable idea of the amount of fat that will be permanently enhancing the buttock region.

Greetings. It is still early in the postoperative period.  There can be some loss of fat over time.  In general, your result seems positive.  Be patient as this is not the final result yet. Best wishes.

It looks like you have a pretty good change from your preop photos. Usually by about 6 weeks what you see is pretty much what you get.