Sounds like a good load, Yogi, but a bit soft for a 4 bore. The powder charge you note is about right for a 10 bore, although your 'shot' load is around 1,700gr. weight.

Sorry, can't help with balls. When Taylor built a 4 bore underhammer gun for a chap, we made the first mould by grinding a spade drill into a heli shape, then bored two pieces of hard maple 1/2 ball depth with it, thus making a mould. It lasted for a dozen balls for testing. 1" Aluminum plate could be bored using the same tool to make a quickie mould. Vice grips can be used clamp it together, aligning up the corners to square the holes.