Where does one get a developer CD?
Not sure.. I bought mine ages ago (i own both the 1.2 and 2.1 revisions). Your best bet would be to try weirdscience, kicksoft, formatt etc. etc.
And does it still apply for OS4 dev?
Most of it will still be apt. The OS4 API is backward compatible, so the data on the 2.1 DevCD won't become entirely obsolete... however it has been said that an updated DevCD for OS4.0 will be released. This should explain the new library system, updated graphics libraries, etc. etc.