I have been trying to find the fourth Stone of Resistance, which is supposedly obtainable right after you finish the Lord of Bones and get Soul Splitter. One of the online guides says it's under the Leviathan Gorge bridge. I believe that the route to the stone is the same as the route to the three chests that show up on my map on the north side of the bridge. On the west side of the south end of the bridge, I found a path that goes to the bottom of the bridge, but it dead ends.
There should be some Death Grip hooks to help you maneuver to the other side of the underside of the bridge. And yes, this is the spot where those chests are. --- Mom and Dad don't look so hot these days, but my future's lookin' good.