For background info:
-- there should be no need to use WinRAR at all,
-- the IK Product Manager ("IKPM") is the recommended tool for downloading, installing, registering and authorizing all current IK Multimedia products; it should make these activities much easier.
Anyway ...
Yes, SampleTank 4 instrument files have the extension .st4i. And their sounds (samples) are in the corresponding .pak files. Similarly, SampleTank 3 instrument files have the extension .st3i (as do Miroslav Philharmonik 2 instruments and others).
I cannot remember which download files contain the instrument files.
Please note that the instrument files contain relative paths to their .pak files so the two sets of files must be in the correct relative folders. This FAQ should help you with the correct folder hierarchy:
https://www.ikmultimedia.com/faq/index.php?id=1239