iOS 9.3 is coming in the Spring and will introduce Night Shift as well as a number of other new features.
Fourth-Generation iPad Models Now 'In Stock' at Many Online Apple Stores
The immediate availability appears to extend to a number of Apple's online stores around the world, including the U.S., Canada, Australia, New Zealand, Hong Kong, Japan, Korea, and Singapore. Shipping estimates remain at 3-5 business days in most of Apple's other online stores for countries where the device has launched, but it is not unusual for changes in Apple's shipping estimates to roll out to different regions on slightly different schedules.
The iPad mini remains in short supply, with Apple continuing to quote shipping estimates of two weeks for new orders of Wi-Fi models. Cellular-capable models appear to remain on track, with Apple quoting "Mid November" in the United States and "Late November" in other countries.