Our setup looks identical to yours, and the width between the plates is exactly 4 inches (103 mm), BUT.... it does narrow down where the forestay chainplate is located to 3.375 inches (87 mm).
We also protected the deck in that area from abrasion by the chain. We used a 3 inch wide strip of kevlar fabric set in polyester resin that is holding up very well, but also considered a 4 or 5 mm thick plate of Delrin instead of stainless. A lot easier to machine on the boat :), and very durable to the kind of sliding abuse that the chain inflicts. It also looks better as it wears.
Bill & Karen
Annapolis MD, USA