...able to use "use net::FTP"...
Could be that you're spelling Net::FTP with the
incorrect case. Are you sure its perl 4.0 (in that case,
I'm not sure what version might work, you might check the
change log (and then the CPAN archives if its
not in the authors CPAN directory) for Net::FTP and see if it goes back that far).
(update: Hmm, the libnet bundle seems to start arount 1997, and perl5
was a few years before that, so you're probably out of luck).
Anyway, if you don't mind my asking, what's preventing you from upgrading perl?
As you probably know, 4.0 is rather archaic :-)