I'm an orthopedic nurse and my specialty is hip/knee replacements. I think a trip to your ortho guy is definately advisable. If it's getting stiff he maybe able to do some light manipulation, you also want to make sure your not developing an infection (even after four years it can still happen though not super common).

If it is time for a revision (fingers crossed that it's not) ask about different types of artificial joints. There are different options out there for young active people vs more sedentary older adults.