nicmar
It is possible to get games running at 60fps on iPhone4, but it requires going old school in game development. High/low priority objects. Update times. Make sure to take advantage of few Sprite objects and use the Frames/Animations of Sprite objects rather than more distinct sprite objects.
Don't overload with trig every tick. if you know your angle is changing by x per tick then do that.
So unless your ready to be 32 LEAN AND MEAN your better off not targeting any weaker device. because it's really easy to make bad performant games in C2. I've seen even weaker than iPhone4 android games that run smooth. but it's all about being crafty.
Otherwise stick with IOS8+