Android Nexus 4 KitKat
This question is both easy and difficult to answer because there's little official information about the rumored re-release. So everything I say concerning that should be considered an educated guess.


If you buy a Nexus 4 right now, you'll have Android 4.3 (Jelly Bean).

Android 4.4 (Kit Kat) Will be an automatic Over-The-Air update sometime in the middle of November (So about a week or two).

If, at any point, you buy a Nexus 4 from a retailer other than Google, it will most likely come with 4.3, and you will need to make sure it gets updated.
If you buy the (Still rumored, not yet officially announced) re-release of the Nexus 4 later, it will most likely come preloaded with Android 4.4 if you buy it from the Play Store.