4 weeks post-op I fell on my BBL. Can it cause damage? Should I be worried?

Probably not.  I guess anything is possible, but I suspect that the only thing that may occur is a little bit of bruising and swelling. Follow-up with your PS to determine if any damage was done.
Kenneth Hughes, MD, ABPS Board Certified Plastic Surgeon
Los Angeles, CA

Thanks for your question. You may not have any issues apart from getting some bruises or an hematoma. Best is to discuss specifically with the operating surgeon so that he /she can take a look.

Dear MsMarton,
hard fall on your buttocks can cause hematoma formation and other issues. But if you don't have severe pain, swelling or bruising you are probably fine. If you have concerns, I suggest you to discuss them with your plastic surgeon. 
Daniel Barrett, MD
Certified, American Board of Plastic Surgery
Member, American Society of Plastic Surgery
Member, American Society of Aesthetic Plastic Surgery