A big user interface overhaul makes Safari look polished rather than clunky on Windows, builds in better search abilities, and makes good use of the fact that people often visit the same sites over and over.
However, the lack of something like the extensions architecture that Firefox pioneered still means Safari 4 (download for Windows and Mac OS X) is better only than Safari 3, not the competition. More at; http://news.cnet.com/8301-13579_3-10170764-37.html?tag=inside & http://blogs.zdnet.com/gadgetreviews/?p=1654 [Apple announces Safari 4 browser public beta; ‘3X faster’ than IE 7, Firefox 3]