I'm in the process of developing a visually complex app targeted for iOS8 that also requires some CPU/GPU horsepower. It's going fine, and I've tested on iPhone 5 and 6/plus, but developing the autolayout for an iPhone 4S will be very difficult and I don't want to have to create separate NIBs for every screen to support the small form factor 4S. Additionally, the 4S running iOS8 (which it doesn't run well) would provide a poor user experience for this app.
Regarding App Store submission & distribution, does anyone know of a hardware feature available in iPhone 5 and up that would allow me to select against installation on the 4S, so that I don't need to create a UI specifically to support it?