This is an officially supported router, but the ABR-4500 wiki page for it has not been updated.
You simply upload the OpenWrt install firmware via the stock "Firmware Update" page in the GUI.
This has a 5 port VLAN switch so in OpenWrt don't uncheck "Enable VLAN functionality" because you'll soft brick the router.
Fortunately, it's a simple hold of the "reset" in the rear to factory default back to your OpenWrt install.
The stock Luxul firmware is good and stable because it's actually OpenWrt. It's just handicapped so no firewall, cron, adding packages, root access, etc.
As of now once you flash THERE IS NO UPGRADE. This thread explains it.
Can someone that has access be so kind to put this on the ABR-4500 wiki for the community?