Not good enough! You should definitely request a ballot every election year.
True, Federal law now states that a ballot request is valid for 4 years. And true, some states - like California - allow permanent absentee registration.
But bear in mind... our overseas ballots are administered by 3,000 different local election authorities (count 'em! 3000!) and each one has its own understanding (or misunderstanding) of the law. Better safe than sorry -- send a new ballot request every election year!