sometimes depends hw u use ur car.. if u drive in city with alot of jams. den better to change now coz u mentioned its oredi 4years+ even its only 51k, the condition of the belt might nt be in good shape.. if you are frequent highway user, then continue to use it till 70-80k...i just changed my waja timing belt even it clocked 45k+ but the belt's age is 5years. the condition of the belt is quite bad n luckily it did not snap.. . the choice is yours...if me just change it for the sake of less headache and major $$$$