1. Create a new A4 document
2. Run the script “PlaceMultipagePDF” on that document with the desired PDF. Place PDF on: “1”, when prompted.
3. Create and run a new script, info found here: https://forums.adobe.com/thread/741023
4. Remove page 2 (should be an empty page)
5. File > Document Setup > Tick the Facing Pages – OK
Then I had to adjust the layout of the first page.
Also, add a final single page, copy/paste the first page there and adjust it to get the back cover correct.
Now I could print it as booklet, woho! (Then the printer said the job was to large, but that’s a different problem)