Jeff,

There are numerous issues to understand about splines, and yes, I am one of the traditionalists.

If you are using hardwood timbers with a 1 1/2" shoulder offset for pegs, 4" tenons provide plenty of relish. Just remember that Peter McCurdy has warned us about getting pith in our relish. Best to use two pegs in each tenon.

If you would like more relish (I like dill) you could use 1 1/2"x 5" tenons offset to each side of a common layout line. If the timbers are not housed (assumed by the 4" tenon length you mention) you actually will remove less wood from the post.

Good luck with your project.

Rudy