The MUD engine is a daemon just like sendmail or telnetd. If it bound itself to that port correctly, then regardless of any silly firewalling goop that mandy tried to do, you should be able to telnet to it from the machine itself on 127.0.0.1:4000. If that doesn't work, I'm skeptical that the mud daemon is actually running.
To have it running automagically from boot you'll have to add a line to /etc/services like:
and then have another in /etc/inetd.conf like:
muddaemon stream tcp nowait root /usr/sbin/(something) muddaemon
where the something is whatever the executable for the muddaemon is. Then you should be able to telnet into it... I think. I've never done that either, and honestly very few people have. Offhand, what flavor of mud? LP, Diku? Tiny? kicking that habit nearly took me a year...