I saw this billboard on a local freeway and it begs the constant argument about when to use less and when to use fewer. Fewer should be used when you are talking about things that can be counted. Grammar Girl calls them “count nouns.” Less is used when you are talking about things that cannot necessarily be individually counted. Grammar Girl calls those “mass nouns.” So when you’re talking about four years, and you can count the years, it makes fewer the correct choice. See the whole blog post about when to use less and fewer here.