the Apple iPad mini sold out "across the board" and that has been followed by word that the same thing can be said for the fourth iteration of the Apple iPad. Orders for the new full-sized tablet placed with the online Apple Store won't be shipped for a week. Keep in mind that this is the Wi-Fi only variant of the device as the Wi-Fi + Cellular models won't ship until the middle of next month as planned.
Originally, it was believed that a "refreshed" version of the third-generation iPad was to be introduced alongside the iPad mini. The changes were to include a faster processor, lightning dock connector and global LTE connectivity, basically what we saw with the fourth-generation Apple iPad. It could be that Apple considered a device that runs twice as fast as the then current model, to be more than just a "refresh".
source: Apple, 9to5Mac via BGR