Is it possible to do a sliding genioplasty after filler was placed in the chin area 3-4 months ago?

If you do not have any hi res pictures from the side of your chin before the filler was placed there, I recommend to wait until the filler is completely absorbed or dissolvfed with hyaluronidase. Even if an experienced facial plastic surgeon may asses the right chin implant for you, this will be based on assumption and no clear measurements. Therefore it is essential to do a thorough planning from the "original shape" of your chin.
Hope this answer helps!

I would assume that you have before pictures from your filler treatment which could serve as the guide for the chin bone movement. If not then it is best to either get the filler dissolved or wait until the fillers has been resorbed before proceeding to chin bone surgery.

 Hi, I have performed many facial shaping procedures, including Chin Augmentation with dermal fillers or silastic chin implants, for over 30 years.  When the chin is weak, this creates an imbalance making the nose appear larger, the mid face top heavy, the lower face looks short, de-emphasizes the lips and allows early formation of a "double chin".  
 Proper placement of a silastic chin implant adds forward projection to the chin thereby creating harmony and balance to the lower face.  Using the same incision, liposuction can be performed to reduce the fat and further shape the neck.  Excess skin, from below the chin, can also be removed through the same incision.  
 I have found that placement of a silastic chin implant, through a small curved incision under the chin (also allows excess skin removal) to be very safe, quick, highly effective and far less invasive than a sliding genioplasty (requires extensive tissue dissection, bone cuts and placement of metal screws and plates to secure the cut segments of bone).  I perform chin implant surgery in 30 minutes or less, often using a local anesthetic alone.  An "expert facial shaping surgeon" should have issue placing a silastic chin implant even if the fillers have been previously injected. 
Hope this helps.