The 6 inch will actually recoil more than the 4" because the barrel dwell time is longer. Now you are also getting into Perceived recoil and thats where the 4" will Seem worse.

4" has more muzzle blast, that's noise and flash and the pressure wave you can feel in your sinuses. The 6" will seem more pleasant because of reduced blast and flash. But if you measured recoil objectively a longer barrel produces more.