As of RISC OS 5.28, the Raspberry Pi 4B is officially supported by RISC OS.
The Raspberry Pi 400 requires a RISC OS 5.29 ROM dated 15 November 2020 or later.
The Raspberry Pi port has a stable version available in the Software Downloads section. Users with an adequate backup strategy in place may wish to try the new features present in the beta version instead.
The RISC OS Roadmap covers items which are not specific to the Raspberry Pi, and the bug tracker covers defects in existing components.
Software information: Raspberry Pi: Firmware has details of the firmware required for the Raspberry Pi 4, along with details of the changes needed to config.txt.
If you are starting from scratch, one way of constructing a boot SD is given at Installing RISC OS on the Raspberry Pi 4?.
For more general information on Pi 4 operation, see Using RISC OS on the Raspberry Pi.