I installed RC-FTPd on my 4000/OS3.9, and I use it on OS4.0 also. I think the setup GUI is excellent and easy to implement.
It has a couple of bugs, which Robin Cloutman (author) is unable to fix because he lost the sources.
1. IBrowse won't log on from another machine. Strangely enough, I have no trouble logging IBrowse onto other FTP servers, so it's an interaction between IBrowse and FTPd. Other browsers like Opera, IE, Konqueror and Mozilla have no trouble with FTPd.
2. There is another problem with early versions of Magellan's FTP client - somehow filenames are not terminated properly and "can't be found", usually because there is unwanted text on the end of the request line. An update of Magellan fixed that one.
All in all, I'd recommend FTPd.
tony