Yes, 4" wide, because that's how long I make my handles, and it supports the handle only near the outside edges. That way it won't tip, even if there is a significant disparity between their strengths, relatively speaking. If it's WAY off, it will still tip.
That will get you a good start. Following that, I made inserts that drop down into the cradle that can be swapped out or moved to support it anywhere an archer might prefer his bow hand pressure point... or used to FIND the dynamic balance point of any bow in which it's unknown.
And yes, you're right, it makes you want to test all of your previous bows... but I'm not sure I'd recommend it. I ended up retillering and refinishing a bunch of them.... finally had to stop... cuz I preferred to build new ones. I still do it once in a while though. Sometimes curiosity just gets the better of ya.
Btw, I do all of my bows this way... glass bows too.