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.
For the intended application you won't see anything really from a longer rod...less piston weight but heavier rod...short rod works well with pump gas and tends to be peppier throttle response. Just how I would do it from experience.