Of the five numbered flatirons, the Fourth is the highest, reaching almost the summit of Green Mountain. It is a large structure composed of three slabs separated by two gullies that descend from the northwest to southeast. The gullies cannot be seen from Boulder; from there, the Fourth looks like a monolith. Though the east face of the Fourth Flatiron cannot be recommended as highly as those of the First or the Third, those seeking remoteness and sense of adventure will find them there.
Start from the Chautauqua parking lot and walk along the fire road to the Bluebell Shelter (0.75 mi). From there, follow the Royal Arch Trail to Sentinel Pass. Descend from the pass. When the trail starts rising again you go past the Fourth Flatironette. Continue for about 50 yards until the trail goes by the foot of the Fourth Flatiron. Looking up west you will see a large cave, and much closer, a small cairn built on a small ledge some fifteen feet off the ground.
This is a long route (about 1000 feet) by Flatiron standards. We did it in nine pitches. It could be done in as few as seven, or as many as twelve.
From the Royal Arch Trail, climb to a tree, then traverse left to the south face of the first slab and reach a large cave that offers a convenient belay station. Exit the cave on the left, climb a short headwall on good holds, and gain the ridge of the first slab. Follow the ridge to the summit of the slab and downclimb west into the gully that separates the first from the second slab.
Cross the gully and attack the second slab aiming for a crack that runs in the middle of its east face. The crack is more like a V-shaped groove and towards the end, it widens to a gash splitting the southern and northern summits of the second slab. The gash can be (barely) reached with one 60 m pitch if one starts climbing the second slab directly across the gully from the summit of the first slab.
The gash hosts an interesting hanging garden
that leads to the second gully. Cross this gully and attack the third slab angling up and left to reach a crack that ascends almost to the summit of the Fourth Flatiron. Continue past the end of the crack on easy rock to the summit. Part of the crack can be avoided by traversing on a ledge to the right edge of the face, climbing a few easy meters on the edge, and then traversing left on another ledge.
From the summit of the Fourth Flatiron to hiking terrain to the west is only a few feet. However, these few feet are overhanging. There are three options: lower yourself from holds on the western edge of the summit; jump; rappel. We found a sling, threading a small tunnel, left by a previous party, and chose the third option. Once on the boulders west of the summit, scramble north past the Challenger, and then descend east in a wide gully. Stay close to the right (south) side of this gully to reach the Royal Arch Trail at Sentinel Pass. Resist the temptation of descending either on the south side of the Fourth (hellish bushwhaking awaits you there) or between the Fourth and the Challenger (not much better).
It is possible to escape the route after each of the two lower slabs. It is advisable to do so by hiking north, rather than south to avoid desperate bushwhacking and poison ivy.
We placed stoppers (5 through 7), Camalots (0.75 through 3), red and yellow Aliens, and a couple of mid-sized hexes. Some pitches, especially the ridge of the first slab, take lots of pro, while others, namely the cracks on the second and third slabs, are quite runout.
External LinksMountain Project's Fourth Flatiron Page
The guidebooks by Roach and Rossiter cover this climb.