How to use iPhone 6 back up on iPhone 4s?
It may be that the 4s is an iOS version behind:

If the iPhone 6, was/is on iOS 10/11(how to check it https://support.apple.com/en-us/HT201685), there’re no way to do this, if not, try update the iPhone 4s to iOS 9.3.5 (how to update https://support.apple.com/en-us/HT204204), and then retry restoring the iPhone 4s from your backup.