As I understand it:
-- Syntronik and MP2 install into folders within a main SampleTank 3 sounds library folder, creating that folder if it does not exist (that is, if ST3 itself and its own library is not installed).
--When ST4 is installed with its sounds library, you can have multiple disk paths - one to the ST4 library and one to the ST3 (that is, Syntronik and MP2) library.
If you want to keep them all in one place on disk, then create an "IK Multimedia" directory and install into "SampleTank 3" and "SampleTank 4" folders in there.
I guess that the Syntronik, MP2, American Acoustic installers always look for the "SampleTank 3" folder.
In the ST4 Browser, you will probably see "SampleTank 4" and "SampleTank 3" at the top level; in the same way as ST3 shows "SampleTank 3", "Philharmonik", "Syntronik" etc.
But ... this is speculation.