I've never needed to set unixboot, ever. The 4K will always try to bring up the Unixware as long as the UIXBIOS file was loadable. If you give the ADS a soft (like sbcsu's RESTART command) it will definitely try to start Unixware. If it can't do it, unixboot won't help.
I would either put a V24 cable on it and see what prompts you get, or give it that RESTART and then check hista for Unixware messages, there will definitely be some.