Maybe it is a gun that I had previously owned, naw I never had a 4 incher. Anytime I shoot fixed sight guns they shoot low for me. I suppose because I hold them pretty tight. It's easy to fix shooting low though, just whittle the front sight off some. You might try shooting some fast, maybe +P, 110s. The theory is a light bullet causes the gun to recoil less and gets out of the barrel sooner so the gun is pointed lower when the bullet leaves. Maybe some beefy rubber grips that don't allow the gun to torque so much could help.
Trouble is you could have a lot of dough tied up in trying this and that and still never get a cure. And if the 110s work, is that the load you will always want to shoot in that gun? That's why I never buy fixed sight guns anymore. I just can't figure this craze for Vaquero Rugers.
I remember a Skeeter Skelton story, He was an armorer or something for the Boarder Patrol and they were using fixed sight Smiths. He said he fixed shooting right or left by bending the front sight.