Flying 9 days after BBL?

Sitting on the pillow should be fine, as long as you are well hydrated, taking an aspirin prior, flexing your feet while seated, and getting up every hour to walk the aisle.  All of these steps should minimize clotting risk, but discuss this with your surgeon
Best of luck!
Dr. SubbioBoard-Certified Plastic SurgeonNewtown Square/Philadelphia, PA

Generally, I recommend that patients travelling from out of town not return home until 2 weeks post op. This will allow for adequate initial follow up time so as to minimize the likelihood of a major complication arising after your return trip. This will obviously vary from one surgeon to another.
However, I believe it is always prudent to consider surgery as close to home as possible. In reality, it is impossible to predict when a complication will arise (if we could they would never happen). With that in mind, dealing with a complication is always easiest without the challenge that distance creates.
With regards to your specific question, there are a number of points to consider with regard to post operative travel. Lengthy flights have their own inherent risks (including immobility & thromobotic risk). Offloading is crucial to the success of fat grafting. A pressure mitigating pillow and intermittent walks and positional shifts should be adequate.
As always, I would recommend following the advice of your board certified plastic surgeon (ABPS) and when flying take the proper precautions for positioning and mobilization so as to limit any risk of serious thrombotic events as well offload your newly grafted buttocks. 

Thank you for your question.  You are not going to be able to stand or walk around for the entire 4 hour flight. I would recommend sitting on a BBL pillow for the duration of the flight.   You will need to stand up and stretch from time to time.  Hope this helps!