'S.B.' - The "4" is 400 yards.

The "B" is actually a separate setting and stands for "Battle Sight" and is at about the 375 yard mark on the leaf.

In theory, with the leaf folded in the 'down' position and the slide set at 'B', the rifleman would aim "low" at an enemy "Line of Battle", from point-blank to 500 yards.  The trajectory of the 200 grain, .30-40 cartridge, would register hits between the ground and the height of a man, out to 500 yards.