Essential Buzz.....
You say it showed 3.99Gbs when 'Properties' was demonstrated by the salesman; was that on his PC, or yours (presumably laptop, if travelling)? ...If the former, presumably you have tried this on yours since?
Plug in the USB and try Administrative Tools > Computer Management > Storage > Disk Management, for a more detailed examination of the drive; not only size but if status is "healthy".
If no luck, but it appears OK, you could try defragmenting it with the standard Windows Defragmenter. ...'Error checking' of the volume also available via My Computer > USB Drive > (right click) Properties > Tools (tab)
As a last resort it's not been unknown for a USB drive to carry a virus. ...Whilst a lot of people furiously scan there C Drive for such, not come across many who do likewise with their portable drive. ...So, nothing to lose scanning it.