I had the hand on Dreamweaver CS4 for more than one month now, I’ll share with you what really helped me in my everyday life. To shorten, for people afraid of my English: if you didn’t make the move to DW CS3 du to the lack of improvement you will be happy with DW CS4!
There are a lot of new features, that I’ll let you discover in details directly on Adobe Labs my favorites are below.
Related Files: Displays all the documents associated with your current page.
This feature is really useful, imagine your are working on a php file, that include a lot of other files (php, CSS, JS). Dreamweaver will offer you a shortcut to these pages, so you are now only one click away from the files you need to edit.
Subversion integration: You can update your site to get the latest versions of its pages directly from within Dreamweaver.
Definitely a must have for every web projects!
That was the features that saved me the more time, after the deception of Dreamweaver CS3 I am really happy with the new version!