thanx for sharing your concern.
also nice to know that you have started taking care of your teeth.
now four years is a long span of time,you havent used your retainer all this time.now if you try to fit old retainer, presently it might not help, plus it can cause damage too on continuous usage.therefore donot worry take it to your dentist
explain your problem to them and i m sure with minor adjustments it can be used again,condition being your teeth havenot shifted much.
if you want you can get fixed type retainer also.you can inquire this from your dentist and get it fixed.