For both the Pi 4B and Pi 400, the easiest way to install RISC OS is to use Raspberry Pi Imager (Windows/macOS/Linux) as follows:
See below for usage instructions.
You will need the following items:
Unpack the RISC OS Pi archive and use a utility such as Win32DiskImager (Windows) or Etcher (Linux/macOS) to copy the image to the MicroSD card. This will create an SD card in a suitable format, but only the first 2GB of the card will be usable by RISC OS.
With some monitors (particularly 4K monitors) you may find that the desktop is displayed at a low resolution (e.g. 640×480). You can increase this to 1920×1080 via Configure > Screen in the usual way.
1 If you connect it to the other port you will get a picture but no sound.
2 It can take 30-40 seconds for the boot-up progress screen to appear. You should be at the RISC OS desktop some 50 seconds after powering-on (roughly twice as long as it takes on a Pi 3).
If you experience problems starting RISC OS, check the Boot-up issues page in the first instance.
See Using RISC OS on the Raspberry Pi for information applicable to all models of Raspberry Pi.