Ultimately, I think the worst 4th-tier unit isn't the Catapult, but the Cavalry, although it's a matter of quality.

Cavalry, with proper support, can take down neutral creatures in no time. Just recently, in the 1/8 finals of the World Championship 09, I messed up because I had 3 Catapults in my army. First, they had a speed of 2(!), and second, they dealt an average of 70 damage. Meanwhile, 1 Cavalier can deal that much damage, if not more.
The fact that Catapults don't reach their target immediately doesn't really matter, because they can't do much anyway. Champions, on the other hand, can do something; they're not angels, of course, but they can clear out neutral creatures.