What features of iOS 7 are available to iPhone 4S users?
TUAW posted this breakdown of new features and the devices upon which they will work when iOS 7 was announced:


  Panorama -- The photo feature Apple introduced last year will be
  available on the iPhone 4S, iPhone 5, and fifth-gen iPod touch. iPad
  users are out of luck here.
  
  Square/Video formats and swipe to capture -- These features will be
  available on the iPhone 4 and above, the third-generation iPad and
  above, the iPad mini and the fifth-generation iPod touch.
  
  Filters in Camera - This Instagram-inspired feature will only be
  available for the iPhone 5 and the fifth-generation iPod touch.
  
  Filters in Photos - This will be available on the iPhone 4 and above,
  the third-generation iPad and above, the iPad mini and the
  fifth-generation iPod touch.
  
  AirDrop - AirDrop in iOS 7 will only work on the iPhone 5, the
  fourth-generation iPad, the iPad mini and the iPod touch.
  
  Siri - Siri in iOS 7 has new voices, a new interface and a whole lot
  more functionality. Users trying to take advantage of the latest and
  greatest from Siri can run it on the iPhone 4S and above, the iPad
  with Retina display, the iPad mini and the fifth-generation iPod
  touch.
  
  iTunes Radio - Apple's long-rumored music streaming and recommendation
  service will run on any iOS 7-compatible device.


Since this announcement, Camera filters were added to the iPhone 4S as well.

The iPhone 4, iPod Touch 5th Gen, and iPad 2 and up models fully support Control Center, Notification Center, and Multitasking.

EDIT: Here's a helpful chart posted by Gizmodo: