[Question] App Design, resolutions

[Mister K]

Hi there, a quickly about resolutions for App Designs.

I am currently designing my first (full feature) app and have been re-searching quite a lot lately regarding resolutions. People tend to suggest many different resolutions and of course you have the "problem" of Android/iOS/Windows and it's devices.

What is the resolution I should focus on? Should I start somewhere in the high end and then simply downscale the elements in Photoshop?

I have started my first design of the App at 640x960 321PPI resolution but have also seen people suggesting 1136x640 for iPhone 5.

PS:

Initially the App will be developed for iOS focused on running on iPhone 4 to 5(s) and if the App is to become larger (or an investment) and worth developing on the Android side we will be doing so.

This is something I was never taught at College (stupid print based design >.>)

Thanks in Advance!