I had a lumbar spinal fusion operation 4 years. what could be causing fluid drainage from the site of operation?

It could be a sign : Of an infection, a leakage of spinal fluid directly or from a psuedomenigelcele or an old seroma that leaked through or even an old hematoma that liquified and is now leaking out. Is this new or has it been ongoing? See your surgeon

CSF leak, Infection: If you are having drainage from your incision after surgery four years ago you need to see your surgeon asap to for work up and likely MRI and/or ct. This can be a serious condition like CSF or infection that can be life threatening if left untreated.