IPhone internal memory used mysteriously?

I have an iphone4s 16gb. I'm supposed to have 13.5 gb space available. But 4.6 gb of space has been blocked out. In iTunes it is showing by the name of others. In settings, that 4.6 gb has no mention. My total data is 5.7 gb and I only have 3gb space available. I had reset all data on this iPhone and synced it with a new Apple ID.

The 4.6GB is the OS.

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