I don't think it's exaaaaactly what you're looking for, but there's The Trick Saga (http://archiveofourown.org/series/4291) which has the four of them up against the apocalypse with Dean/Cas and Sam/Gabe. The smut isn't so smut smut smut as it is love love love (if that makes sense....). Regardless, it's a great read!