A HAT (Hardware Attached on Top) is an add-on board for ROCK Pi 4 that conforms to a specific set of rules that will make life easier for users.
A list of working HATs tested by radxa and the community.
Zwave module by Z-Wave.Me. It uses uart2 TX/RX on the 40PIN GPIO. Tested working on ROCK Pi 4 by disabling the serial console on uart2. See discussion on the forum.