See Thumper for the general location. From Thumper, hike and scramble 100 feet almost directly west into a gully on the south side of the 4th Flatiron slab. Here you will find a south-facing, vertical, huecoed wall. This route climbs the first set of obvious huecos to a ledge with a tree. The climbing is fun on big holds. We were going to bolt it to make it a fun warmup, but winter and the bolting ban the next year left the route as is. There are several more viable routes along this wall and they would be adventuresome undertakings on natural gear.
Good addition! I believe this climb is on the south-facing wall of the second piece of the Fourth Flatiron.
I presume you get down by rapping off the tree? One could probably continue up the Fourth Flatiron, but then it would be hard to get back down to the base of the route.