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.
ROCK Pi PoE HAT
ROCK Pi PoE allows to provide both power and network connectivity to your ROCK Pi across a single Ethernet cable. Refer PoE_HAT page for more info.