Lost 3 GB memory from iPhone 4s after update to iTunes 11.4.018
For my phone - the space was being taken up by the download and unpacked iOS 8, which was waiting to be installed. Once I installed the software - the "missing" space appeared again, and the bar displaying my drive capacity was reflecting correctly.  

I would check that first - to see if it already downloaded the update.  The link will say "Install Now" on the Settings > General > Software Update page in iOS.  

Regards, 

Ed