iOS 7.1, released earlier today, includes support for existing iPad models along with support for two unannounced models, labeled iPad4,3 and iPad4,6. It is possible that these new model numbers represent minor hardware revisions for existing iPads that will be introduced to the market silently, as a more substantial upgrade would push the model numbers higher.
On occasion, Apple has implemented new hardware components for its iOS devices without a public announcement and these minor changes often go unseen by consumers as they tend to introduce very minor improvements in efficiency. For example, an Apple TV update in January of 2013 revealed a new Apple TV, which turned out to be a minor update with a smaller A5 chip. That change was not obvious to Apple TV purchasers, but it did warrant a model number jump to 3,2 from 3,1.
Existing iPad Air models are numbered 4,1 and 4,2, while Retina iPad mini models are numbered 4,4 and 4,5, suggesting one new iPad Air for the 4,3 designation and one new Retina mini for the 4,6 designation.
As noted by 9to5Mac, it is also possible that iPad4,3 and iPad4,6 represent iOS devices that are optimized for networks in other countries.