I've looked closer. Hope it still helps even though not timely.
Ejector is twisted and not so bent on the 4 vs 3 as well as the housing is different due to grip I suspect.
Trigger bar is different in that the 4 has the dimple on the FP safety cam. 3 has no dimple.
Barrel caming lug is different on 4 as it appears to be shorter/ longer between 2. Easily seen with both barrels upside down next to each other. Sorry no pics. I expect due to more case support on 4 barrel chamber.
Also noticed on 4 that it is very steppy going into battery in the last 1/4" of travel Gen 3 very smooth and deliberate. 4 seems to stick a bit and then pop into battery. Sometimes hangs out of battery if done very slowly. Brought concerns to customer.