America Online (AOL) has quietly released the latest version of its client software--version 4.0--after two years of testing. AOL users will be able to download the latest version from within the online service's current software, while a Web download version will be available next week. AOL 4.0 includes a modern user interface with more graphics and a "channel" format. The AOL 4.0 Email client include spelling and grammar checking, along with other improvements. Most notably, users can change their onscreen identities without shutting down AOL and restarting