Hi, Welcome to Healthcaremagic!
You haveNOT mentioned your age. It seems that you had some ligamentous or tendon tear/injury
4 years back, for which you didnot take proper care & it has not still healed completely. Also you should know that tendons & ligaments have poor blood supply & heal poorly. Due to continuous activity, you might have developed post-traumatic osteo-arthritis
of knee. I advice you to consult an orthopaedic specialist ASAP. Good luck.