You can refer to this one to flash your board with a specific rover firmware. Because it is experimental in PX4, so you can’t use the default firmware.
I think for your case, for Pixhawk4, you can use px4_fmu-v5_rover.
I tried it previously with my Pixhawk6c and Pixhawk6x, and it’s working just fine for version 1.15.2, with airframe aion robotics r1, by setting the SYS_AUTOSTART to “50003”