Sorry seeing now that 15.4 is the first native Apple silicone version. The latest Intel version I could install is 15.2... Installed. Again no luck opening Preferences.
oooookay....
I'm not a newb, just a part time idiot.
Apparently I had installed the Tourbox software, but being a laptop hadn't disconnected it from my iMac yet. Forgot I had installed the software. The Tourbox menu bar wasn't there as it hadn't connected yet.
Connecting the Tourbox I can now safely open my preferences. So there is something about the Tourbox app that kills Premiere (if opening prefs) when the Tourbox isn't connected. Good to know. I will contact Tourbox regarding this.