From what i've read and experienced, L1 and L2 are both fairly random...dont track Jumpman all that much at all, and are normally not a threat...nor can you really influence them much at all by moving.
L3 and L4 I would say are about the same...but like Mitch said, L4 can bounce at sharper angles resulting in faster moving wild barrels as they bounce from one girder to the next.
L3 and L4 track your current position and will zig zag down towards you, bouncing to your left and to your right, trying to stay above you and keep you within it's path of being able to hit you. You can kinda control these so to speak. From what i've experienced, the wild barrels will bounce much sharper to the right, than to the left. If you're right of a wild barrel as its hitting the girder above you, itll bounce right, so I try to be to the right of the wild barrel as it is hitting the girder above me, which will force the barrel to bounce right of me, as I am then running left. You can even steer them off the screen by doing this when you're on the far right hand side of the screen.
It seems to be much more dangerous to be to the left of a L3 or L4 wild barrel as it it bouncing off the girder above you. If you're to the left, it can bounce left and land right on you, or bounce left, then bounce off of the girder you are currently on, to the right, and hit your leg/foot...so it has essentially 2 chances to kill you.
So stay to the right when it's hitting the girder above you. The wild barrel hack is a good way to test this out...or develop your own method of avoiding these evil L3/L4 wild barrels.
Level 5 wild barrels track your current position and attempt to cut the distance between you and it by 1/2. You will notice that if you stand still, the L5 wild barrels will just slowly fall towards you at a fairly even rate...getting closer and closer to you as it bounces off each girder. You'll also notice that if you stand still and are toward the right hand side of the screen, they can't hit you...because they aren't trying to exactly track Jumpman...but rather are cutting the distance between you and it by 1/2 at each girder bounce. It's been said that this logic was put into place so that on the girders where you are running left...the L5 wild barrels will land right on you if you aren't paying attention to them...which most novice players might do. They track you just enough to land on your head if you're running toward them, in certain situations.