So, I have run into some instances as of late where I was forced to use different revisions of the same hardware (two FortiGate’s that match model wise but are different hardware revisions, you know, cause Fortinet likes to evolve things half way through a lifecycle) in high available (HA) clusters. This is easily done with a few tweaks at the CLI level to enable it to properly function. I ignore 4.x version code because if you are still running that you are going to have a bad time in general.

execute ha ignore-hardware-revision enable
execute ha ignore-hardware-revision status

This will make the hardware work with ease in an HA cluster. Take note, you still need the same model, this just helps with variances in hardware revision between the two!