340 Stroker Rotating Assembly

Brian Hafliger said
Better combo would be a 4" crank and 6.125 rod...

I'm curious what benifit a 6.125 rod brings to a 4" crank other than a shelf piston? Looking at moving up to a 4" stroke, I was still looking to run a 6.200- 6.300 rod to get a shorter piston, less wight and less friction.