Hello Yuhong!
Take a look at once: oliver.gatinee.free.fr., La Belle 1684 - Modélisme d'arsenal - Oliver Gatine. Under "Modéles" you will find different models with different scales. They were built by talented specialists. Glenn Grieco has chosen a different interpretation for Texas A&M Model than Jean Boudriot.
We are not building for a museum, but only for ourselves. First the family and friends, then the shipmodel. With the equipment, with barrel, anchor, bed, stove and other things you have created a beautiful handcrafted model.
Your question. Yes the keel should be 4,0 mm thick. But CAF used the same wood thickness as for her ribs 3,1 mm. Otherwise CAF would have had to start a new board for the keel parts.
Successful modelling
Thomas