Of all the guidelines for college preparation, those connected to foreign languages tend to be the most ambiguous and confusing. Selective colleges and universities usually require at least two years of language course work in high school. Many schools, however, "highly recommend" or "urge" applicants to take three or even four years of a language.
For admission to the top liberal arts colleges and top universities, taking four years of a language is probably a good idea. Scoring a 4 or 5 on an AP language exam is also a convincing way to demonstrate proficiency in a second language. To learn more, check out this article on foreign language requirements.