Safari 4 may unexpectedly quit if “Glims for Safari” by MacHangout is installed.
Uninstall “Glims for Safari” as described on this website.
That MacHangout URL is long gone, but the Wayback Machine grabbed it, and the page included a link to an uninstaller.
I had no recollection of Glims for Safari, but the Wayback Machine is helpful there, too.