The 4S will get everything that the 6 and 6+ get that does not require the 6 or 6+ hardware... I imagine there will be a list like D mentions above soon.
Not only hardware requirements, but it should be addressed as location on where certain services will work and will not...
Only info I can get is from Apple themselves.
Siri is available on iPhone 4s or later, iPad with Retina display, iPad mini, and iPod touch (5th generation) and requires Internet access. Siri may not be available in all languages or in all areas, and features may vary by area. Cellular data charges may apply.
Turn-by-turn navigation is available on iPhone 4s or later, iPad 2 or later, and iPad mini with cellular data capability. Cellular data charges may apply.
FaceTime calling requires a FaceTime-enabled device for the caller and recipient and a Wi-Fi connection. FaceTime over a cellular network requires iPhone 4s or later, iPad with Retina display, or iPad mini with cellular data capability. Availability over a cellular network depends on carrier policies; data charges may apply.
Safari full-screen view and tab view are available on iPhone 4s or later and iPod touch (5th generation).
AirDrop is available on iPhone 5 or later, iPad (4th generation), iPad mini, and iPod touch (5th generation) and requires an iCloud account.
Health is available on iPhone 4s or later and iPod touch (5th generation).
AirPlay is compatible with the second-generation Apple TV or later. AirPlay Mirroring is available with iPhone 4s or later, iPad 2 or later, iPad mini, and iPod touch (5th generation).
Refers to the total number worldwide. Not all content is available in all countries.
iCloud requires iOS 5 or later on iPhone 3GS or later, iPod touch (3rd generation or later), iPad, or iPad mini; a Mac computer with OS X Lion v10.7.5 or later; or a PC with Windows 7 or Windows 8 (Outlook 2007 or later or an up‑to‑date browser is required for accessing email, contacts, and calendars). Some features require iOS 8 and OS X Yosemite. Some features require a Wi‑Fi connection. Some features are not available in all countries. Access to some services is limited to 10 devices.
Find My iPhone enables you to locate iOS devices only when they are on and connected to a registered Wi-Fi network or have an active data plan. Some features may not be available for all countries or all areas.