Make iOS app compatible on iPhone 4,5,6,6 Plus

I have developed an app for iPhone 5. But when I run it on iPhone 4, lower part is not visible, and when I run it on iPhone 6/6 Plus, it shows only on a part of display, leaving other part white. (Which is logical actually). So my question is - what should I do to make my iPhone 5 app, to scale up or down depending on device it is install on. Or in other words, how to make it compatible with iPhone 4, 6/6 Plus?

Thank you.