This is the procedure to use if you want to run RISC OS 5.27 on the Pi 4B but need to construct the SD card image from Windows/macOS/Linux (i.e. you haven’t got another RISC OS machine).
You will need the following items:
Unpack the RISC OS Pi RC16 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.
Card preparation is now complete. Remember to dismount the SD card before unplugging it.
You will need EITHER a USB-C hub with Power Delivery (this may only work with newer Pi 4 board revisions)1 OR an alternative method of powering the Pi 4 (e.g. via the GPIO pins) so that a conventional USB hub can be used.
Problems you may experience:
1 If you have board revision 1.1 or 1.2, you may have to supply power by another means (e.g. via the GPIO pins).
2 If you are still experiencing problems with low screen resolution, have a look at the Video options page on the Raspberry Pi website.
See Using RISC OS on the Raspberry Pi for information applicable to all models of Raspberry Pi, including the Pi 4.