8GB model iPhone 4S is only showing 5.4GB capacity (4.2GB available)
The operating system (OS: iOS 8.x) is using up space as well and you're not able to use this space. The phone comes with 8GB, yes, but minus the space used for the OS. I think Apple got into problems when they advertised with 8GB/16GB etc. while people could only effectively use that space MINUS the space used by the OS.