The all-in-one Archimedes were the 3xxx series: A3000, 3010, 3020.
https://chrisacorns.computinghistory.org.uk/Computers/A3000....
https://chrisacorns.computinghistory.org.uk/Computers/A3010....
https://chrisacorns.computinghistory.org.uk/Computers/A3020....
That would have been a far more obvious and better fit.
The Pi 400 is based on the Pi 4, so 4000 would have fit, but the A4000 was a classic 3-box design with a SoC:
https://chrisacorns.computinghistory.org.uk/Computers/A4000....