steel
Steel shot has a deserved reputation of lacking range and penetrating power on waterfowl, which, if you are a duck hunter, you already know. The immediate solution for that was to shoot larger shot, and heavier payloads, hence the intro of the 3" and 3-1/2" magnums. Steel shot is quite hard, so patterns frequently can be tight for a given choke versus how the gun performs with lead.
All that said, I suppose, lacking any other loads, your magnum #4steel should perform about like #6 lead.......I think. Due to the hardness of steel shot, and the severe constriction of most turkey chokes, they are marked "FOR LEAD ONLY" so I would NOT run steel through a dedicated turkey choke.
Pattern your gun and see how it does with the #4steel, a modified choke might surprise you on how it performs. Given decent patterns, I'd say magnum #4 steel could reliably take a gobbler to 35 +/- yds, but I would not want to stretch it further.