As with the Safari 4 betas, Safari 4 passes Acid 3 tests with a 100/100 -- the first (and only cross-platform) browser to achieve this level of standards compliance.
The big news for users of the Safari 4 betas is that the tab orientation has been changed. In the beta, tabs were on the top of the application window, now they are below the URL field, just like in previous versions of Safari. I'm a fan! I also like the snazzy updates to the "loading" notification and the RSS indicator.
Safari 4 has way too many features for me to list, so I'll just point you to Apple's page that details everything. The HTML 5 media and offline support, CSS 3 web fonts and CSS animations are some of my favorites.
Download Safari 4 here and then come back and tell us what you think! Remember, any third-party InputManagers like Saft will need to be updated if you want to use them with Safari 4.