Ummm... the AmigaOS 4 prereleases all come with a text-based FTP client called FTP.
I used to use ncFTP with AmiTCP under AmigaOS 3.0 when I had my A1200 on the Internet and it worked fine for downloading off of the Aminet. I'm not sure how a non-anonymous FTP should be done using ncFTP but there should be a command line option that will let you enter your user name and password.
-edit-
The easiest one to use in this situation would probably be FTP-Mount. I'm not sure how to do non-anonymous FTP with it either though.