Some new features of Safari 4 includes top sites, cover flow, history search, tabs on top, nitro engine, windows native look and feel, full-page zoom, developer tools, phishing and malware protection etc. The top sites features will enable you to see your favorite websites at a glance and open them with a single click, while with the help of cover flow feature will can flip through your site history or bookmarks just like you flip through albums in iTunes. With Safari 4 you can easily create and manage tabs. Tabs offer a great way to have multiple pages open at the same time in a single browser window. Even you can drag and drop tabs.
More Info: http://www.apple.com/safari/whats-new.html