* iPad 2 = iPad Mini = iPad 3 = iPad 4 = iPad Air = iPad Mini Retina
* iPhone 5 = iPhone 5C = iPhone 5S = iPod Touch 5th gen
* iPhone 4S is a "sort of" outlier. If you're using the auto-layout stuff that's available it "just works" on a 5* phone. The exception is apps that aren't widget-based, like games.
* iPhone 6 and 6+ seem to have different logical resolutions, so there's that. But again, auto-layout helps here.
The "different DPI" thing is irrelevant. Get the UI elements to have the same size proportionally and you're set.