Fairy dust is sprinkled on the Gen4 RSA.
I conjecture that having a different grip texture or grip shape makes you no longer a blinded observer, so you are no longer objective in determining recoil sensation. Shooting "Recoil" is as much a psychological as well as physical perception anyways. For example: If you wore no hearing protection, the same gun, and ammo would give you more perceived "recoil".
To the extent that I am aware (worthless of course), there are no tests of the actual spring tensions comparing Gen4 and Gen3 RSA's. You need an instrument to test this. Most of us assume they are different since they look different. Actual pressure per unit length may not be different at all, or insignificant for most humans to perceive. Who is to say the Gen4 RSA is not actually lighter?