Apple on Monday released Safari 4, the next version of its Web browser for OS X and Windows users. The updated browser will also ship with Snow Leopard once the planned OS X 10.6 update arrives in September.

Safari 4 has been available as a public beta since February, when Apple unveiled the new version of the browser. The new version features the Nitro Javascript engine, which is aimed at improving Safari’s speed.

During Monday’s WWDC keynote, Bertrand Serlet, Apple’s senior vice president of software engineering, touted the speed improvements to Safari, claiming the updated browser can execute JavaScript nearly eight times faster than Internet Explorer 8 and more than four times faster than Firefox 3. Safari 4 can also load eight pages three times faster than IE and Firefox, according to Apple’s figures.

Safari 4 will run as a 64-bit application in the forthcoming Snow Leopard update. That will further boost the performance of the Nitro JavaScript engine.

Also in Snow Leopard, Safari 4 will offer a a crash-resistance feature by running plugins as a separate process. The leading cause of crashes in OS X comes from browser plug-ins, Apple says; in the new version of Safari, if a plugin crashes, only that part of the page will be broken, while the browser will remain open.

Other improvements in Safari 4 include a Top Sites feature that gave users a visual preview of frequently visited pages; Full History Search for finding titles, Web address, and text of recently viewed pages; and a Cover Flow view for browsing bookmarks. Safari 4 Beta’s tabs-on-top feature, where Web page tabs were located at the very top of the window, is gone in the final release. Instead, tabs have returned to where they were in Safari 3, immediately below the bookmarks bar.

The new version of Safari includes HTML 5 support for offline technologies as well as support for advanced CSS Effects. According to Apple, Safari 4 is the first browser to pass the Web Standards Project’s Acid3 test, which looks at how well a browser adheres to CSS, JavaScript, XML, and SVG standards designed specifically for dynamic Web applications.

Safari runs on Mac OS X 10.4.11 or later and requires Security Update 2009-002. Apple says Safari 4 is available for download at its Web site.

Updated at 11:25 a.m. PT to include more information on the Safari 4 update.

Updated at 12:58 p.m. PT to note the removal of the Tabs on Top feature.