whats the difference between 4th and 5th generation ipad air

See here for a comparison of iPad models: http://www.apple.com/ipad/compare/

Technically there is no iPad Air 4th generation. The fourth generation iPad is the iPad with Retina Display. This current model full size iPad is the iPad Air. The iPad Air is thinner, lighter and has a faster processor than the iPad with .retina Display.

iPad 4 vs iPad Air

http://www.trustedreviews.com/opinions/ipad-air-vs-ipad-4

iPads are classified by generation.

There is no such thing as a 4th generation iPad Air.

The iPad air as such is a 5th Generation Model. That is, its the 5th type of iPad that has been released.

The 4th Genertation iPad was not an iPad Air. It was a regular iPad.

The main difference is the iPad Air is lighter than a 4th generation iPad. Though there are a few hardware differences.

Now that Apple is selling iPad 4s again one difference is $100.00.