(Windows Version)
After a long time struggling with Sampletank 4 not showing any instrument sounds when I clicked on an instrument on the left side (though all the instruments appeared fully without locks or download symbols), I finally found where the scan data was located and was able to achieve a new rescan that made all the sounds appear.
In the folder \Documents\IK Multimedia\SampleTank 4 you will find the scanner database file "instruments.db". Delete or move this file out of that folder, return to Sampletank 4 and click Rescan All, and all your sounds will appear.
I hope someone else finds this immensely useful. I look forward to finally enjoying Sampletank 4!
Terry
P.S. - I do not know where the Mac version places this file, but perhaps it has the same filename and you can locate it if you have encountered this same problem of a corrupt database file being the issue.