To take a look at new features, here’s a link to What’s new in InDesign CS4. And here’s a link to What’s new in InCopy CS4. These topics list the top new features, with links to detailed information about each.
Note that many links to video tutorials, other Help documents, and white papers are still not functional, and the search doesn’t really work yet. And commenting isn’t turned on yet. This should all be fixed when the CS4 products actually ship.
Here’s a link to the new Community Help page, which is still a work in progress:
InDesign Community Help
If you have any suggestions for improving Help, email me (I spelled it out here to avoid spam): bbringhu at adobe dot com