I am almost certain that uninstalling the ST3 software does not uninstall the ST3 sounds library. After all, they were installed separately.
But you wont get back much disk space. And if you ever need to Import legacy instruments, you'll need ST3 as ST4 does not have an Import feature.
If you understand what you're doing you could try this (not endorsed by IK Multimedia.
-- rename the ST3 sounds library folder,
-- uninstall the ST3 software,
-- if that uninstall was set up to uninstall the ST3 sounds library, it would not find it,
-- when complete, change the ST3 sounds library folder back to its previous name.
Check that all is still OK in ST4 SE (not "es"
)