A 4 bore lead ROUND ball weighs about 1500 grains, yes. I shoot a 4 bore blunderbuss and my bore is 1.050" in diameter and the round ball I use is 1.025. The round ballsI use weigh in at just about 1500 grains.

Mine is a 25 yard or closer gun so what works for my situation is not really relevant to yours.

However I would say a .015 patch using a ball .02 to .025 smaller than your bore would work well. Or a .02 patch and you could use a round ball that is smaller than your bore by .04 I would think. I find that a patch that has lube on it swells some more and gives a tighter fit than just looking at the numbers.

And yes I would use an over powder wad, I would use 1/8"+ thick cork wad, perhaps thicker to help absorb some of that recoil.

What distances do you intend to shoot this 4 bore rifle at?

Those extra long 32 inch barrels and all the heavy hardware should make this a 14 to 16 pound gun I would guess.

Those 4 bore cartridge pictures are truly awesome. I may have to get me a 4 bore cartridge gun some day.