The release coincides with the start of the company's Worldwide Developers Conference, which also saw the announcement of a new iPhone and iPhone OS, the introduction of a new laptop and refresh of others, and a peek at the next version of Mac OS X.
Safari 4 introduces a number of new features, some of which will be apparent to the user and some of which are behind the scenes.
The new browser is also the first to support HTML 5 and new CSS standards. HTML 5 includes audio and video tags that allow the development of multimedia sites that don't require plugins.
The CSS support enables Safari 4 to display CSS animations and special effects, as well as to download custom fonts needed to render a page as the designer intended.
Apple claims that Safari 4 is the only browser to have passed the Web Standards Project Acid 3 test of its ability to render advanced Web sites.
Those are the behind-the-scenes enhancements. For a personal tour of the interface changes, see Page 2.