what thickness cardstock is best for what
For the ribs and the keel, a matte board would be great but you can laminate to the thickness of the original by using cereal boxes or even cardboard but if you use card board, be sure to cut lengths of paper strips to cover the exposed corrugated edges.
For planking, Cover the body with strips of paper like you would in the double planking method but using 120lbs weight paper or thicker if you can find it. Poster board will work for this and cut into strip to do the planking. Doris over at papermodelers has a ton of tricks for this in her wonderful build of the HMS Victory.
For the mast I would use dowels and cover with light 20lbs weight paper. The same method for the yardarms. For rigging, use upholstery thread, which is very tough and is not fuzzy like regular thread (They use it for sewing shoes).
I have not built a four mast ship but it worked great for the Swift boat.
Good luck and hope to see your works soon.