About the "update - unauthorise - register ":
-- Doing the software update will install the latest version of SampleTank 4 on your system.
-- Other users have reported problems with the ST4 library not being loadable in ST4. The cause seemed to be that the software was Authorised with the ST4 CS Serial Number. The solution was to Unauthorize ST4, Register the new ST4 Serial Number then Authorize ST4. That will pick up the new Serial Number and make the sounds loadable.
ST4 can load its own library and also the SampleTank 3 library (just put the paths to both libraries in ST4's Preference Settings / Sound Content. You will see that the Edit page is different for the ST4 instruments and for the ST3 instruments; the "ST3 audio engine" is embedded in the ST4 software.
Features in ST4 and ST3 are broadly similar. If you want you can just use instruments from both libraries in ST4.
The ST4 and ST3 sounds libraries should be in different folders. For example:
L:/IK Multimedia/SampleTank 3
and
L:/IK Multimedia/SampleTank 4
and both of those paths would be in ST4's Sound Content list.
For more details have a look at this FAQ:
https://www.ikmultimedia.com/faq/index.php?id=1239
As for further choices it all depends on you - what have you got so far? What sort of things do you want to do? And what is your bonus level?