Here is more info:
http://www.androidcentral.com/amazon...kindle-fire-os
But there is one concern, Amazon's app store is frequently running behind the Google Play app store when it comes to updated apps, and that makes me think we may run into a number of apps that won't work or be updated for awhile. Don't get me wrong, I want the Kitkat/Sangria version, but we may have to wait for some Amazon app store apps to catch up again. One concern I have in particular is a review I read on the new HD 6 that says Overdrive doesn't work on it. I think the apps will catch up, but it may take awhile.
edit/update: I have not got the actual letter yet. Does it say "some features" of Fire OS 4 or actually a system upgrade to Fire OS 4 for 2013 Fires?
Fire OS 4 is available on the new Fire tablets, with some features coming to the 3rd generation Kindle Fire HDX and Kindle Fire HD (released in 2013) as part of free, over-the-air Fire OS updates starting later this year.
https://developer.amazon.com/public/...n-Fire-Tablets