Just replaced the formatter board in my old USB-only LaserJet 400 so that it has networking. This means that it is now a m401n. One of my other machines uses Linux Mint and it works absolutely fine with hplip and 'hp-setup'. It is able to find the right "*.ppd" and the printer works.
However, on Arch Linux it detects the printer on the network via 'hp-setup' but when I click on it the PPD file field is yellow and says the following: "(Not found. Click browse button to select a PPD file.)". I did as it instructed and search through the "/usr/share/ppd/HP" directory. I found "hp-laserjet_400_m401dne-ps.ppd.gz" but this seems to be incompatible with my model (m401n) since it just closes the PPD file selection screen and continues to display the same yellow text field.