What is these lumps in my buttock 4 months BBL post-op? Should I be concerned?

Great question!  These little pea like lumps are most likely fatty oil cysts.  When the fat is injected, sometimes they form these cysts have have fatty oil in them.  They are of no medical issues if you don't have problems with them, but if they do start causing issues, then your surgeon can drain them if they are big enough.Best of luck and hope this information helps!
- Dr. Roger Tsai  @DrRogerTsai
Board Certified Plastic Surgeon

my best guess is you have several small areas of fat necrosis. This represents fat that did not get a blood supply during the healing process the first week after the BBL and now has turned into scar tissue you could also possibly have oil cysts  also caused from fat necrosis. Follow up with your plastic surgeon treatment options are direct excision or drainage if they contain liquid  like oil from fat necrosis. Best, Mats Hagstrom M.D. 

Hello, Thank you for your question! 
Sometimes, when the fat is harvested to the (buttock) can form fatty oil cysts. If if doesn't hurt or if is not bothering you, they don't a problem But if they are big enough you can visit your surgeon for a consultation and ask him to drain them, only if they are big enough. 
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When grafting fat, PS strive to place fine threads of fat. Occasionally, clumps occur and overtime fat can die, scar and form lumps. They can resolve over months and typically shrink gradually.As long as you have no true issues, waiting is fine.