I was able to get Cisco VPN Client 4.0 (REL) working under Panther. I did another install, this time to a Firewire drive with an existing OS X 10.2.6 installation on it. The 10.2.6 disk did not have any previous VPN installs on it.
After the upgrade/install, I installed the Cisco VPN Client 4.0 (REL), copied my existing profile (.PCF) to /etc/CiscoSystemsVPNClient/Profiles/ and restarted. (I am using group authentication.) Cisco VPN client worked after that. As usual, my problems seem to be self-inflicted.