NEW 4" CRANKS $250.00 any good?

Well,piston speed for a 4" stroke at 5500 rpm is 3666. Well within limits for stock pistons actually. As for imposing rpm limits on a cast crank..I'd rather impose one based on rods and rod angle/piston material. Cast cranks have the ability to absorb tosional vibration better than forged. But they flex more...