Zeros and Ones 

Sounds interesting, but I'm a bit confused...Is this the 4th pitch of a previous aid climb or a new route?

Chris--
It's the original fourth pitch of Practice Climb 101, which tackles the huge roof left of the Sooberb crux roof. Erickson left it out of Rocky Heights, since as you know he only included free climbs. The omission was perpetuated in subsequent guidebooks.

From: "ELDORADO: A Climber's Route Guide" by Pat Ament (1975) pages 105-106:
Practice Climb Number 101. II, 5.7, A3.
First ascent in 1966 by Larry and Roger Dalke.
Find the start to the Side Wall. Immediately to the left (north) is a steep, narrow face (or buttress). Free climb to an inside corner and nail 70 feet up to a ledge. Belay. Nail the overhang above (on the left) and free climb to a ledge. Belay. Now go up a slab to a big ledge [and belay]. Nail an obvious overhang (just left of Sooberb) and free climb to the top.

Perhaps it's a bit presumptuous to re-name the pitch. However, it's a great pitch of free climbing on all natural gear-- it will no doubt get climbed independent of the rest of the route more often than not. And when pitch two was free climbed, John Allen proposed calling that pitch "Practice For What?".

Really interesting pitch. Climbs much better than you'd think. It could use a lot of cleaning up and will probably improve as it sees more traffic.