The "R4" image will only work on 4-inch iPhone running iOS 7 or later. On older version of iOS it will use 2x image.
Example. If you use images like this:
you will see:
- iPhone 5 iOS >= 7 - number 3,
- iPhone 5 iOS < 7 - number 2,
- iPhone 4, 4S - number 2,
- iPhone 3GS - number 1,
- iPhone 5C and 5S - number 3.