Image assets not working correct in iOS

I used image asset for Image management in my app. My app supports iPhone4,iPhone5 & iPhone6, iPhone6 plus devices.

but when I uses image assets then what resolution need for @2x image by which both iPhone4 & iphone6 works?

My try for view background image

Case 1 if I put 640 X 960 resolution image for @2x then works fine for iPhone4 but not works in iPhone6.

Case 2 if I put 750 X 1334 resolution image for @2x then works fine for iPhone6 but not works in iPhone4.

Help me to solve this

Thanks in advance