imj: Castle aren't relying on "back alleys like the Drobe forums" to make announcements. Check out their web site, they have the press release there.
From the anouncement I see no reason to assume that Castle are forcing end users to switch to RISC OS 5. They have stated that they are commited to supporting 3rd party licencees, even competitors. These 3rd party licencees all use RISC OS 4 +Select. Once this has been settled I would imagine that ROS 4 etc would still be developed (whether by ROL or Castle) and supported. Quoting from the Castle statement "Castle is in discussions with the shareholders of ROL to find a way to offer RISC OS 4 and Select to users. It is expected this will lead to a satisfactory conclusion in the very near future and offer a clear way forward for the whole RISC OS community."