Unfortunately I think this one is going to need a hands on physical assessment to add any real insight into what may or may not be happening. After 4yrs with no relief of discomfort you might consider a revision of the graft. But again one would need to assess the pain as you get the pain. i.e. provoke it during a physio session and then assess what movement or otherwise might be done to reduce it on the stop as a means to isolating the causative factors. The ACL should not normally be an issue going down hill or downstairs as it is not under much tension. It is more of an issue going upstairs or running up hills. BUT if the graft is overtight then perhaps it could be playing a roll. Would be nice to work out where the pop is coming from. Could be joint line, patellofemoral joint, foreign body in the knee etc. My advice would be to go to a good knee/ortho/sports PT and get them to work with you to provoke all symptoms and try to evaluate the cause a little more.