Internally, Google Engineers work "by dessert" [1], the version numbers are decided by someone further down the product pipeline. With an instance like this, the developers most likely made a number of updates to the same branch of the OS (Jelly Bean), that someone in marketing/product management felt were significant enough to warrant a new 4.x number. At the end of the day though, it's still the same Jelly Bean core, so the next desert is reserved for another actual OS branch.
[1] - I believe JBQ said this to the media once, but I can't find the source now.
[1] - I believe JBQ said this to the media once, but I can't find the source now.