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Four years is quite long enough to be brainwashed.

Your mother in law might very well have been match making, one never knows.

Any traditional woman will be disparaged and they might already be involved so save any bank statements, investment account statements, etc.

He will be advised how to hide all his assets before he drops the bomb so be wise as a serpent and harmless as a lamb.

Best wishes to you. Maybe you could convince him to see a church marriage counselor and see if that might help.