High posted (knee wall) capes are nice stuctures, but, providing a blanket answer as far as how high the post can be is difficult. I will depend on the size of the posts, the pitch of the roof, how much snow you get there, etc.
But, a collar tie may be able to help a little bit with the forces. Practically, the posts would need to be fairly high and the tie quite low, almost like a full second story.
It comes down to a stiffness and joinery concern as much as anything else. While the timbers may be strong enough (they won't break) for a 4' eave wall and a collar tie 2/3rds of the way up, the timbers won't be stiff enough to keep the wall from spreading too much. Also, this is relying a lot on heavily loaded tension joinery, which requires a lot of wood removal of the post / rafter to make a good joint, thereby weaking the member at its most stressed point.
If the floor plan allows, stout down braces from the post to the tie beam, this certainly can help a bit.
Joe
