What is an iPad air 4,2?

Helping my brother buy a case for his iPad (it was a gift and he doesn't really know what he has). He says the description is "iPad air 4,2" and it is 9.4" x 6.6". I can't find anything that is called an "iPad air 4, 2". I am seeing iPad 4 and iPad air 2. Don't want to get something that won't fit. If it helps, I think he received it as a gift Christmas, 2014, so I am guessing it is a Fall, 2014 model? About time he opened it:) I know this is a really dumb question, and maybe I could find out more if I actually had the iPad in hand, but I would appreciate any help you can give me. Thanks!