For AI servo there are two basic types of settings -
1. Setting a single focus point (or expanded focus point) and tracking the subject by keeping the focus point on the subject. This is what I use most often. It works well for soccer, track, and volleyball (that is what I shoot). Basically, when you are trying to stay with one moving subject that is in a crowd of potential subjects - the camera wouldn't know what to focus on, so this one is best for that kind of shooting.
2. Selecting a group of focus points and letting the camera track the subject within that grouping. As the subject moves, the camera will switch focus points. In this setting, you can select the starting focus point, and then the camera takes it from there. If your subject is moving around but largely staying within the frame, this would work. If the subject is fairly isolated this makes sense to use. I don't use this one much.
There are a number of other settings in the custom settings for fine tuning how the AF system behaves. For example, if the focus point loses the subject you can adjust how quickly will the camera focuses on something else. For example, if you are tracking a bird in flight and lose the subject for a second, you can set the change to be slow so the camera will not focus quickly on something in the background while you are trying to get the subject back under a focus point.
It can get complicated with all the custom settings, but at the core of it, it is pretty simple as I described it above. The 1D Mark IV manual is on the web and there is good detail on how that AF system works, which might also help.
It works just like before.
