(Answers below, but blease ask one question per thread in future.)
This command uses a Windows Search query to find things rated 4-starts:
Find QUERY rating:"4 stars"
You shouldn't have the "/O" there. The /O etc. just indicate the type of argument and aren't part of the argument/command-line itself.
It's just SetAttr META=tags:Test
I don't know if there is a way to add a tag to the existing tags via the command line. Send in a feature request if nobody else here can come up with a way.
[quote]Is it possible to create an button, which displays all files with user-defined search words?
Example: A button, which shows all files tagged with "test" or/and 4 stars in a virtual (search) folder.[/quote]
Yes, using the Windows Search stuff mentioned above. This thread has some discussion and useful links.