Perhaps a stupid question but if one already owns a copy of the full SampleTank 4, is there any point in downloading this free CS version. Are there additional patches that aren't already included with the full version of SampleTank 4?
You wouldn't need SampleTank 4 CS itself (the program and plugin) but the 4GB+ sounds are available to you as a SampleTank 4 user and they are new (and quite good!). You can download those in your Sound Downloads in your IK User Area.
Edit - also meant to say you might also be interested in the SampleTank 4 Editor that you get free since you have SampleTank 4. That will appear in your IK User Area as well.
Yes, any SampleTank 3 or earlier sounds can be used in SampleTank 4. Any earlier sounds must be imported into SampleTank 3, and once imported, they can be loaded directly into SampleTank 4 and used with all of the FX and features of SampleTank 4.
Do note, SampleTank 3 sounds will use the SampleTank 3 engine in SampleTank 4, so sounds will not have all of the functions of the SampleTank 4 engine, but you will still have the benefit of being on a later software which will be updated moving forward.