On the other hand, a 4 foot cube (I assume it's 29 tall, yes?) *might* be a good option for some tangs. As long as the rockwork was built in a coral bommie formation, and the fish had full clearance to swim unobstructed around the live rock pillar. I still don't know if I would consider this for a shark, but there are more knowledgeable shark folks in the aggressive fish forum whom you might ask...
I've seen some tanks like this that look outstanding, but you have to be careful. Best if it's a custom tank that's drilled and overflows in the center. Tanks like this that have equipment and rock on the back wall can wind up looking cluttered, but that's just MYO.....your mileage may vary.