The trial of Indigo 6 should work, but I'm not positive that it tries to copy over the Indigo 4 prism files on install (it does copy the Indigo 5 prism files, but I don't recall if the logic looks back further than that).
So shutdown the Indigo 6 server via the Indigo->Stop Server menu item, then make a copy of this folder:
/Library/Application Support/Perceptive Automation/Indigo 4/Preferences/Prism Reflector/
to:
/Library/Application Support/Perceptive Automation/Indigo 6/Preferences/Prism Reflector/
Next, download and run the
Indigo Perm Fixer app.
And, finally, restart the Indigo 6 Server. Does it work? If not, then follow the steps on the Perm Fixer app wiki page and paste the output window results into a reply for me so I can see what is occurring. Also copy/paste the Event Log results showing the prism errors.