Steel will penetrate paper and most plastic wads that I have seen, I have just come to accept that steel shot is going to make my barrel look ugly on the inside.The barrel is modern, made by Ed Rayl (so, not an antique yet!). If I use steel shot, would I need to use a shot cup or paper cartridge to protect the barrel, or is it fine just to use traditional card/cushion/shot/card?
A nice Ed Rayl 4 bore might as well be an antique.
I'd look for a cheap beater import gun or have a T/C bored out smooth and make it your dedicated steel shot garden gun.