Unless you can find them at Dixie Gun Works, I don't think you will find too many sources for 4 Ga. equipment. And, for most of us, buying pyrodex, or any other substitute will be much more costly than black powder. As to loads for the gun, you do what you have to. Its your gun, arm, and shoulder. I have a friend who was shooting a custom made .69 cal. slug gun, shooting a 2 pice bullet, weighing 1760 grains, in front of 350 grains of powder. The gun weighed 100 lbs. Even with that tremendous weight, he told me that you would dislocate your shoulder if you did not tuck that stock in hard.
If you are just trying to see how much pain your shoulder can take, why not just stand behind a cannon with your shoulder pushing against the breech when it fires? If you are shooting a larger bore gun to get more shot in the air than with your 12 ga, then what do you need all that recoil for? The benefit of shooting 20 ga. loads in a 12 ga. smoothbore is that you get no recoil in the latter. The benefit of shooting 12 ga. loads in a 10 ga. shoothbore is you get no recoil in the latter. because the shot column is wider and shorter in the larger bore gun, the shot stays together in flight better, and reaches the target closer together, so their are no gaps for a bird to escape through. The advantage of shooting 12 ga. loads in something like a 4 bore gun is that there will be no recoil, your patterns will be better, and other than the weight of the gun, there is no down side in cost of powder, shot, or noise.
All those advantages are gone when you start talking about feeding your testosterone, by using large and unnecessary quantities of powder, and shot to prove how tough you are. Why all the self doubt? Who are you trying to prove something to?