I would think that given a competent shooter, decent ammo, and a solid rest, a full cylinder load should stay under 4 inches pretty easily. With a better shooter and a load the gun "likes", it might get tighter -- a lot tighter. A six-inch barrel gives a long sight radius. I know I fired some really nice groups with .38 or .357 revolvers, back in the days when I was practicing regularly and had younger eyes. PPC shooters used to shoot coffee-cup sized groups at 50 yards fairly regularly.
If the gun is in good shape and matched to the ammo, 6 shots under 2 inches @ 25 yards might be achieved. I have several "keeper" targets with groups this small or smaller, some shot with 4" revolvers, so it is possible. Guns do vary, though, so it's not a sure thing.