The root should contain the contents of the 4.0 public beta folder in the root of the zip, and not the contents of the 4.0 public beta folder in the _MACOSX folder, right?
Edit: Tried them both with no difference, so nevermind.
When I reach stage builder, it acts as if I have just plugged in an empty SD card, and nothing boots.
Root of the SD card contains the contents of the "4.0 public beta" folder inside the zip, with the boot.elf at root level. It also has an automatically added "system volume information"...