As to ill effects of steel in the older, softer barrels -- this interests me, also. When I've got to shoot non-tox from an older Superposed or Auto 5, I try to always shoot Tungsten-matrix, but it's gotten so expensive as to be almost prohibitive, at the same time the performance of steel shot continues to improve.
I've been told -- but I don't know if it's true -- that modern steel shell are far safer for some older barrels than the early steel, largely because the shot cups hold the shot all the way out the barrel, so that there's little (if any) contact between the shot and the barrel. The cups are better designed to open and disperse the shot after barrel exit, which is part of the reason so many duck hunters use true improved or even skeet chokes for decoying ducks. I know that I do, and I'm still surprised to see a tight pattern out on the water when I finish off a wounded duck.
Still, I'm not sure enough of all this information to trust shooting steel shot out of my old Brownings, and would love to hear from someone who's a real authority on the issue. I guess your experience with number 4 steel in your Sweet Sixteen may be a pretty good indicator that the risk is low.