Nitrosteel- read your "Wayne" quote, then read your second last sentence. That is how I feel about a conical mould for your 'large' 4 bore. A round ball will provide all the speed you'll need, I am sure. Indeed, you will be hard pressed to shoot a 1,200fps load let alone a load with a conical that doesn't shoot like a garden hose projects water. A conical might get 700fps to 900fps maybe.
This rifle sounds like a good project. Probably not useable for much, but a good project, none the less. Steve has set the bar rather high. I'd prefer to see a nice wide steel English shotgun-style butt plate, though, with checkering. I don't think rubber is going to make much difference in felt recoil. These guns move by the foot when they go off, at least the 50 pound 4 bore did my bro made once. 400gr. of 2F would move it 12" on the bags whetehr your shoulder was against the butt or just your hand - you could not stop that movement.
I would not personally shoot a 20 pound or lighter 4 bore from the shoulder, well, probably not more than once.