The mean piston speed tracks stroke length directly: 4.500" is only 7.4% higher than 4.1875".
For 4,000 f/m it's 2,400 ÷ stroke. 4.1875 is 5,731 RPM.
The instant (max) speed does go up with shorter rod, but again only a few percent.
What does change quickly is max piston acceleration, which more or less ~ with RPM^2 (6,000 is not 20% higher than 5,000 but 44%).
