I just grabbed a nicely priced copy of SampleTank 3, and should be able to run it since I've not upgraded to MacOS High Sierra+ (which includes Catalina). However, I may have to upgrade MacOS at some point.
Question: Does IK provide the SampleTank 4 software, empty - minus samples - free of charge to allow customers to import and use their legacy products?
EDIT: I found the free ST4 Custom Shop version. Does this allow the import of ST3 instruments with full features?