How to let iPhone 6 use Retina 4 2x image size in Xcode 6

I'm using swift for my game, and i need to know how to let iPhone 6 use Retina 4 2x in Xcode.

My images are green-square@2x green-square-568h@2x green-square@3x

My objectif : is to force iPhone 6 use Retina 4 2x image for sprite

i use this code to apply image for my sprite

                    let GSquare = SKSpriteNode(imageNamed: "green-square")

Thank.