Contact Arlington -- customer service -- to see if they've got UL/CSA literature that backs your contention.
Your best shot at winning the AHJ over comes from authority.
This is especially true when the issue turns on a specific product.
A four-gang box is pretty uncommon in residential. With any kind of wiring discretion, my impulse is to design away from such a box.
Obviously, any support scheme is sure to compromise Arlington's depth adjustment scheme.
