4.0 Has made a lot of changes. Including some keymap changes. To help people out here is a video explaining the more common changes: Watch here
If you prefer to read all the changes then Read here
You can also go through the Release notes on the Blender Site
If you just want to read the release notes without all the fancy videos and demos then you can Read Developers Release notes.
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I’ve added some of them to the WIKI:
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