When will the Sony a7iv (Sony Alpha ILCE-7 IV) be added to the list of digital cameras RAW formats supported by iOS 15?

No one outside of Apple knows what plans - if any - they have for adding raw support. They tend not to make announcements in advance. In the meantime:

Let them know you want it: https://www.apple.com/feedback/

You can convert your Raws to DNG using Adobe's free converter

Or you can use an app that has raw support independent of the OS, like the Adobe range.