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Under iOS 6, what features will I not get if I were to buy an iPhone 4 vs. an iPhone 4S today?
What I already know is the new navigation (continuous navigation on the lock screen, if I understood that correctly), which will be available in 5 and 4s, and won't be available in 4.
Anything else?
Apple has a nice iOS page up at http://www.apple.com/ios/whats-new/
In case it changes at a later date, here is the relevant text from the end of that page.
1. Flyover and turn-by-turn navigation are available only on iPhone 4S, iPhone 5, iPad 2 or later, and iPod touch (5th generation). Cellular data charges may apply.
2. Siri is available on iPhone 4S, iPhone 5, iPad (3rd generation), and iPod touch (5th generation) and requires Internet access. Cellular data charges may apply.
3. FaceTime video calling requires a FaceTime-enabled device for the caller and recipient and a Wi-Fi connection. FaceTime over a cellular network requires iPhone 4S, iPhone 5, or iPad (3rd generation) with cellular data capability. Availability over a cellular network depends on carrier policies; data charges may apply.
4. Offline Reading List is available on iPhone 4 or later and iPad 2 or later.
5. Made for iPhone hearing aids require iPhone 4S or iPhone 5.
6. Panorama is available on iPhone 4S, iPhone 5, and iPod touch (5th generation).
7. Find My iPhone and Find My Friends enable you to locate iOS devices only when they are on and connected to a registered Wi-Fi network or have an active data plan.
iCloud requires iOS 5 or later on iPhone 3GS or later, iPod touch (3rd generation or later), or iPad; a Mac computer with OS X Lion or later; or a PC with Windows Vista or Windows 7 (Outlook 2007 or 2010 or an up-to-date browser is required for accessing email, contacts, and calendars).
Some features require iOS 6 and OS X Mountain Lion. Some features require a Wi-Fi connection. Some features are not available in all countries. Access to some services is limited to 10 devices.
This text above is straight from the footnote portion / small text at the bottom of the web page.