Their liability is nott necessarily limited to the funds that crossed their accounts.
That sounds like good new for the retail SHs, and being able to keep the divy. Just stick the brokers for the full $4M amount, based on their participation with the fraud part. Far easier than needing for the divy to be declared illegal, and then have the mess of clawing back from discount brokers, and their clients.
The Jewel of the Mind is Colored with the Hue of what it Imagines