The preview version of Netscape 4.5 boasts around 4,000 improvements, including a newly tweaked version of Navigator and a welcome redesign for Messenger, Communicator's email client.  Though many of the changes to the central browsing component are invisible, Navigator has grown a couple of new buttons designed to make it easier to navigate the Web.
Though the preview isn't radically different from the current official version of Communicator (4.05), it gives an early hint of the kind of direction Netscape plans to go in for version 5.0 - currently scheduled for beta release before the end of this year.