If you take your fourth-generation iPad into Apple for a repair, the company may replace your tablet with a newer iPad Air 2 instead. According to an internal memo published by MacRumors, Apple started doing this on March 30, right around when the $329 iPad became available to purchase.
Starting March 30, iPad 4th generation whole unit repairs may be substituted to iPad Air 2 models. Apple's repair and order management tool will indicate for each repair if a substitution will take place. Please note the substitute part’s color and capacity to ensure the customer understands what their replacement iPad whole unit will be.
While you may get a new color and capacity, we also assume that customers with Smart Covers or other accessories for their fourth-generation iPads will need to buy new accessories for the iPad Air 2; we've contacted Apple for clarification and will update if we get a response.
The iPad Air 2 is two years newer than an iPad 4—it's significantly thinner, lighter, and faster, and it support iOS features like multitasking than the iPad 4 isn't capable of. More importantly, it's certain to be supported by the next major version of iOS, while the iPad 4 is more than likely to be dropped from the support list. Apple is likely running out of parts and replacement tablets for the older iPad, while the only-recently-discontinued iPad Air 2 is still available in abundance in Apple's refurbished store.