Hi Guys,
I got past mthat pesky firefight, but got stuck on an error. Anyhoo I searched the web and found this site....it has two patches for Police Quest 4. I downloaded both and so far the errors I had come across have been fixed. Hope it works for you.
http://www.pcgamesportal.de/index.php?site=patc&id=P
GOOD LUCK !!