The "Other" category contains storage that does not belong to the Windows Phone OS itself; for instance, storage which belongs to third-party applications, etc. You've said that you only have a few applications installed, so either they're HUGE applications, your phone is acting up, or you underestimate how many are installed -- I have 56 installed, which use just a hair over 2GB of space; per your screenshot, yours take up more than double that amount of room. More than likely, some of these applications are causing the problem...if you have games installed, each of those will save game progress to "Other"; if you've downloaded maps for use with Nokia Drive, those will be saved to "Other"; if you download songs to your phone via Spotify, those will be saved to "Other"; etc. It's hard to give you more direction without knowing what's installed, but your apps are almost certainly what's causing the space issue.