Hey BJ,
this interested me and I searched for it, but as far as I can see on the Buildbotics Forum, they mainly see the problem that Raspberry Pi altered the connectors positions on the Raspberry Pi 4, so that Pi 4 would no longer fit into the Controller’s case backplate (compare yourself: Raspberry Pi 3B vs. Raspberry Pi 4B). The Ethernet port is now on the right of the USB ports. Also Buildbotics sees increased heat implications with Pi 4 with their current casing.
Other than that, one user reports that he was able to successfully build the Buildbotics-firmware sources on the Raspberry Pi 400 (Pi 4 based). So I would see no reason why it wouldn’t work with Onefinity-firmware sources too – both upgrading the hardware to a Raspberry Pi 4 and upgrading the Raspberry Pi OS to a recent version as well (usually recommended in order not to have a device with a lot of old security issues in your home network).