5'4, 145lbs, 40lbs weight loss, am I a good candidate for BBL revision? (photos)

Your photos are suboptimal for proper evaluation.
You would need adequate fat to liposuction in order to for the fat transfer to make a difference. I’m not sure you have enough.
Dr T

The first thing a plastic surgeon needs to know is what you’re trying to achieve or what you don’t like about your current body.

Weight loss patients typically do not make for great candidates for either a liposuction or fat transfer because weight loss can cause skin laxity.
It doesn’t mean that patients who’ve lost weight can’t have the procedures but the ideal candidate typically has tight skin.
Based on your pictures you have a flat spot on your lower inner buttocks. This area is difficult to get a project with fat transfer.

The surrounding areas will project nicely with fat transfer but the flat spot where most of the pressure goes in a sitting position has connective tissue that anchors the skin to the underlying muscle inhibiting it from projecting by simply adding fat.

If that is your concern then you need to understand the inherent limitation.
Droopiness of your lower buttocks is most likely related to inappropriate Liposuction of the back of the thigh.

We need to see appropriate before and after pictures in order to make a quality assessment.
I always recommend having multiple in person consultations to get an accurate assessment and a thorough understanding of what can or cannot be accomplished. The outcomes of these procedures vary substantially purely depending on who does the procedure.

Look for surgeons with an excellent reputation who have years of experience proven by impressive collections of before and after pictures.

Best,

Mats Hagstrom MD