On http://www.mozilla.com/en-US/firefox/4.0b1/releasenotes/ in the system requirements we link to http://www.mozilla.com/en-US/firefox/system-requirements.html - the list for Firefox 3.6 and there is as example 10.4 listed as supported. But since we dropped support for 10.4 on Firefox 4 Beta 1 this system requirement page is no longer valid of the beta. Same might be happen when we drop support for Windows 2000.
FWIW, it's my understanding that we are keeping support for Windows 2000 in FF4, though I am not sure where I heard that from. I do know community members at the summit made a point to push for Win2000 support as most schools are still on it (and a lot of enterprise users too).