Similarities between the fourth generation iPad and iPad Air:
Lightning connector
iOS 7
Identical camera systems
Siri support
Both iPads are available in 16, 32, 64, and 128GB models, as well as cellular models.
The iPad Air contains a speed bump with its new dual-core 64 bit A7 chip. The iPad fourth generation is negligibly slower at everyday tasks, using the dual-core A6X chip (a modification of the A6, which is found in the iPhone 5 and 5c).
If you're looking to save money, get a fourth generation iPad; refurbished models here at Apple's store are great deals and it still delivers great performance.
If you want the latest and greatest, get the iPad Air; it'll cost you more, but you'll get a refreshed design, faster processor, and more fluidity.