how can I prevent this from causing an error?

[dunfiddlin]

form4 does not fire form1 events unless i do system.application.doevents()

Er yes it does (see my post above). It doesn't appear to because you're stuck in the loop.

form4 code:

debug.print form1.webbrowser1.url.tostring()
'shows www.google.com

form1.webbrowser1.navigate("www.yahoo.com")

debug.print form1.webbrowser1.url.tostring()
'shows www.google.com after i've told form1 to navigate.

system.threading.thread.sleep(100000)

debug.print form1.webbrowser1.url.tostring()
'shows www.google.com after i've told form1 to navigate and slept the thread for 100 seconds.

The sleep stops all operations for the duration! You could set it for 100 years and it still wouldn't have changed!