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.
Very little downside to a longer rod in a performance/ race build. I ran a 6.300 Compstar rod in a W5 motor.
Less sideloading, lighter piston, better rod ratio, in a race motor perhaps better power where the engine lives at going down the track compared to a short rod.
For a strictly street deal, I probably wouldn’t bother