If you don't already have New Vegas (and as much as it pains me to say it) I think now's probably a bad time to get it. FO4 will have more in common with FO3, and it'd be really hard to experience all the good stuff there is to experience in New Vegas in the 5 weeks we've got until FO4. Unless you're not getting FO4, in which case, definitely get NV ;D