[wjy]

In that sentence, "one" is the subject and "in four teens" is a prepositional phrase. When choosing the verb form, we look at the subject, which is this case is singular. So, "is" is in fact the correct form to use.

A more detailed explanation can be found here: http://www.chompchomp.com/terms/prepositionalphrase.htm

[jeggers5]

Surely 'teens' is the subject of the sentence? I have literally never heard anybody construct a sentence like that and use 'is'.

[starwed]

Well, consider how the idiom works. It is asking us to consider a representative sample of four teens, and makes a statement about one of them. That's different than if you said "only 25% of teens are employed".