The AT4's rocket will go wherever you're looking, even when unscoped. One time I was on defense in the lighthouse on Valparaiso (the jungle map) and I saw an enemy Bradley rolling into the village. I didn't have a clear line of sight because there were buildings in front of it. I hardly even saw him, but I could spot him (and my teammates did) so I fired a rocket into the air, over the buildings, steered it down onto the Bradley and destroyed it. Very satisfying kill.
AT weapons in BC2 can be summed up like this:
RPG-7: quite effective against armour. Travels very slowly and has significant drop, which are its biggest disadvantages. The RPG pretty much becomes obsolete once you've unlocked the AT4, unless you use tracer darts a lot. I personally don't - I find it quite difficult to lock them onto a tank or helicopter because they travel so slowly. Maybe I need more practice though. Still, I'd rather have a sidearm.
Carl Gustav: the least effective against armour, but the rockets travel very fast and are much more effective against infantry because of its splash damage. More of an all-round weapon. Can be used with tracer darts. I haven't used the AT weapons a whole lot since the patch, but I heard the CG was nerfed, making it less effective against infantry. As of now I can't confirm or deny that.
AT4: by far the best AT weapon against armour. Unlike the other two, it doesn't work with tracer darts, but like I said, the hand-guided function can be useful. However, you also have to keep yourself exposed for almost the entire duration of the rocket's flight, because it's not a fire-and-forget weapon.
Depending on your preferences, different maps and situations it's often best to switch between the three. Also, all three will destroy a helicopter with one hit.