Kitkat internal memory issue

I am using HTC Desire 816 handset which contains 8 gb of internal memory. In that 8gb i can use only 3.9gb for storing app related files and other stuffs,remaining space occupied by OS. Currently i am facing memory not available issue while downloading through internet or installing new apps. so i checked the space available in internal memory using ESS file explorer and found 400mb space free and 3.5gb space used. But i dont have that much files in my internal memory. So i selected all the files in the "emulated" folder ("0" & "Legacy") and checked the properties which shows total size is 1.8gb. Here my confusion is both the files are of size 1.8gb.But i select both the fiels and check the whole space it is showing 1.8gb only. So where is the remaining space,which file occupying that space,can i delete that files. please help me.