Much of my playing with large bores has been with Elephant powder, which tended to foul more than Swiss. With large charges (e.g. 300 gr in the 6 bore) the olive oil seemed to keep fouling softer than my usual patch lube of neatsfoot oil.

Last summer I won a bunch of Swiss in a couple of matches. At the velocities I wanted in my rifles and my 11 bore shotgun (the biggest gun I've kept around, shot only with ball), it left hard fouling. I switched from neatsfoot oil to olive oil, and found the fouling problem considerably reduced.

Buckstix, what is the rate of twist in your 4 bore? I hadn't considered a conical, so I'm looking forward to hearing how that works out.