Yes, generally RAD is a supported configuration for FB4 Plug-in.
Which version of RAD are you running now?
Actually, I'm asking this on behalf of a potential customer and I'm not sure which version they're using...
Generally, can I tell them that RAD versions based on Eclipse 3.4.2 and 3.5 are supported?
Flash Builder 4 Plug-in was tested with RAD 7.5.4 on Win, so that would probably be the most reliable combination to recommend.