The place to start is the release notes: https://github.com/d3/d3/releases/tag/v4.0.0

And the detailed change list: https://github.com/d3/d3/blob/master/CHANGES.md

I will update my tutorials and examples (and probably write new tutorials) in the near future, and I expect others will, too. But as I now have thousands of examples, I didn’t want to hold the release on updating all of them. My newer examples use 4.0 already, and I’ll be going through the others in the coming weeks.