"Hey Siri" support and possibly wireless charging case alongside AirPower charging mat.
AirPods and AirPower: Everything We Know
Apple May Replace Your Broken Fourth-Gen iPad With an iPad Air 2 Where Necessary
An excerpt of the memo obtained by MacRumors:
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.The fourth-generation iPad was released in November 2012, so when a customer needs a replacement model, it's likely easier now for Apple to just give them a newer and more widely available iPad Air 2 in most cases.
Not every broken fourth-generation iPad will be replaced with an iPad Air 2. The decision will ultimately be decided on a case-by-case basis, likely depending upon availability of each tablet in the affected customer's region.