Many I-4's have two short and two long stacks to even out the power.
I know my 'Busa is that way, as is most GSXR line of bikes.
Without measuring, I believe the RC51 has one short and one long as well for that reason.
At one time, there was a place that made aluminum ones with slightly different intake bell curve. But I can't remember what the site was.
My buddy who has an Aprilia replaced his with aluminum ones, maybe it's the same place.