You can separately download just the release notes from the Rockwell Automation update page.
I got a PPT file a few weeks ago that illustrates the new features in 4.0; you could probably get a copy of that from your Rockwell distributor.
The key points I am looking for is a new security model in FactoryTalk and improved functionality of SE redundancy. Machine Edition sports a new recipe function and (very important to me) the ability to create runtimes for all the previous firmware versions of PanelView Plus.
Other new features include Global Objects, some new faceplate libraries, and the runtime language switching feature.