I think you're misremembering IE's version numbers. ExtJS started up as a YUI extension called YUI-ext circa 2006. IE3-4 were long dead by then. You might be thinking of IE7-6.
I may be misremembering, but my recollection was my then employer started using Ext version 2 primarily since it had the rich widget ecosystem and also supported IE 4 well. We were writing internal enterprise apps and it was a good fit.
As you can guess, that's been quite awhile ago. My memory is fuzzy on the details to be sure.
http://www.w3counter.com/globalstats.php?date=2007-05-30