Some people just don’t want the iOS 8, like me, because it’s not the best system for iPhone 4s.
But Apple likes to make decision for the users. It will auto download the iOS 8 firmware for you if your phone has sufficient space.
Here are some methods to stop it.
For unjailbreak users: If you have a router, disable these two domains in it.
then it will shows this:
For jailbreak users: Delete these files in /System/Library/LaunchDaemons [Please make a backup!!!]