Preinstalled audio, photo and video data size
It could quite easily be Garage Band, if not other app data too. My Garage Band folder at /Library/Application Support/GarageBand is actually 4.8 GB

Source: Delete GarageBand to Save Precious Gigabytes of Mac Storage


  As a rich media application, GarageBand installs relatively large groups of media files to several locations on your Mac’s drive. Based on a clean install of GarageBand 10.0.2 on OS X 10.9.3, key locations of large files include:  
  
  /Applications/GarageBand.app (1.16GB)
  /Library/Application Support/GarageBand (995MB)
  /Library/Application Support/Logic (880MB)
  /Library/Audio/Apple Loops (up to 10GB)*  
  
  *Note that the size of the Apple Loops directory will vary depending on the number and type of loops downloaded by the user. You should not delete this folder (or the Logic folder) if you are using Logic Pro.
  
  Under normal circumstances, however, users looking to delete GarageBand can expect to save around 3GB of space. It may not seem like much, especially as storage capacities continue to rise, but 3GB can make a huge difference for those with small SSDs.