Hi mat, the setup for the CustomUI for v4 is different than for v3, it now goes in the appdata folder which is:
/Users/<username>/Library/Application Support/Moi/

> Where do I put the "customui", "objects" and "startup" folders for V4beta?

If I remember right all of these go in /Users/<username>/Library/Application Support/Moi/ - there will already be a startup folder in there so copy the contents of the CustomUI startup folder into the existing startup folder (/Users/<username>/Library/Application Support/Moi/startup).

Hopefully that will get you set up.

- Michael