Iphone 4
How do you update/restore old iPhone 4 to a new iPhone 4S?
iPhone 4S, iOS 8.1
How do you update/restore old iPhone 4 to a new iPhone 4S?
iPhone 4S, iOS 8.1
The hardware of an iPhone 4 can't be upgraded. An iPhone 4 can only update to iOS 7.1.2
To download this software, set it back to factory settings, explained in this article:
Use iTunes to restore your iOS device to factory settings - Apple Support
Iphone 4