The iPhone 4 cannot be updated beyond iOS 7.1.2
If it is not updated to that, see: Update the iOS on your iPhone, iPad, or iPod touch - Apple Support
The iPhone 4 cannot be updated beyond iOS 7.1.2
If it is not updated to that, see: Update the iOS on your iPhone, iPad, or iPod touch - Apple Support
Your iPhone 4 is obsolete. It has insufficient hardware within it to run anything newer then iOS 7.1.2.
My I phone 4 not updated