"Hey Siri" support and possibly wireless charging case alongside AirPower charging mat.
Apple Beginning to Process Refunds for iPhone 4 Bumpers
Image from forums
Apple has started processing refunds for the iPhone 4 bumper cases to online Apple Store orders. Existing back-ordered bumper cases have also been processed for refund. We expect the process will continue over the next few days. Apple is also expected to provide a method for in-store purchasers to also receive a refund but has provided no details yet.
Apple announced last week that they would be providing free bumper cases to all iPhone 4 customers and providing refunds for those who have already purchased them. The bumper cases help alleviate the signal loss issues experienced by some on the iPhone 4.