though small clicks post surgically are often experienced by patients as the plate rubs against the soft tissues of neck region and patient should not be worried unless some other features like fever, nausea, difficulty in eating or breathing and tingling in limbs or their weakness accompanies.
but in your case the clicks have appeared after 4 years post surgery and they are increasing. the clicks broadly be due to
1. after spinal fusion
the vertebrae above and below the fused areas takes all the load and they get arthritis
early. so the clicks and neck pain
may be due to the degenerative spinal disease
2. can simply be neck stiffness
3. rarely implants get loose and come out of bone and can result in such features.
any how consult surgeon. take care.