Other space on 16G iphone 4s?

I have a 16G iphone 4S and an enormous section of my space is taken up by "other"! Apparently 7.4 of my 16G are taken by other even after I deleted most messages, have only 400 photos (i have had many more), deleted a lot of apps, taken off my email account, cleared internet history and cookies, half my notes, and even restored the entire phone. What could possibly be taking up all of this space?

With my experience on apple products as well as experience repairing and fixing their their firmware issues it sounds to me like that is very much so processing issue meaning its internal i se it all the time the internals of your phone whether it be your logic board board or processing unit may be disrupted and most likely if you already done all the same things i would do to figure out the issue it may not be something you can fix you might be better off getting a new device or calling apple and seeing if they can help you. Hope this helps you out.