sewman,
You make is sound the the #4 is a huge HP plate compared to the #5. Is this really the case? I have never seen a 4 so I cannot comment. In numbers (HP/Torque), how much larger is the #4 than the #5. If we are only talking 20hp, like the difference between the #6 and #5, I don't know why the 4 would be so far out of the question, as you have made it to appear.
Thanks for the information,
Ryan