storage issue

Hello, I have this phone (8gb) since August and never noticed how much storage I have until now because it started notifying me storage was full. Well, it says total space is 4,53gb and without /system 3,60. So my question is, where is the 4gb of storage that I don't see? I was hoping it was occupied by /system but it is not. We're talking about a half of the storage and it's unbelievable that I cannot use at least 5 of the 8 I've payed for.
Any help would be very appreciated!