It's probably just a path issue.
Can you let us know
-- the full path to your ST4 library (the folder containing Instruments and Samples folders etc)
-- the full path to the moved ST3 library folder (the folder containing Instruments and Samples folders etc)
-- the disk paths in ST4's Preference Settings.
This may help too:
https://www.ikmultimedia.com/faq/index.php?category=11&topic=sampletank-4#st4