best to go up a shot size, so rather than use a No.4 in lead you would be us a No. 3 in steel. As with geese rather that use my favorite of No, 3 in lead, I would go up to a No. 1 in steel.
Perhaps the most important thing to remember is that under no circumstances must you use high performance steel cartridges in an old gun. If you were very fortunate the gun might just blow up, alternatively in may result in you or someone else being killed!