Can you get pregnant if your hubby had a vasectomy four years ago?

I was just wondering I am over a week late for my period which is quite unusual, my hubby had a vasectomy after we had our third child was about two months old and he is going to be five in April next year. My hubby did go back for a few check ups and they said the procedure was succesful. Maybe its stupid I am worrying and I am almost scared to buy a test to check, still thinking my period could start any minute now but is it possible or quite unlikely?