Being a Sub-I is very much like being a medical student, except with a little more patient load. If you are not prepared, I wouldn't worry about it too much. They wont expect much of you.
If you are prepared, then you can get as much responsibility as an intern. That was exciting.
By now, you should have developed a method for figuring out differentials, be competent in your physical exam, and know the basic starting points for management. You will be looked at to answer the questions the medical students cannot, know a bit more than they do, and be able to manage more patients than the med students. If med students handle 1, you handle 2. If they handle 2, you handle 3.
To prepare, you should have been paying attention to all your other rotations. The fact you haven't studied anything was definitely a poor decision. But if you are a fourth year, all you have to do is pass, since it will not impact your matching prospects.
Of course, the bottom line will be that every Sub-I is dependent on the attending, the program, and the hospital. You should probably skim through Crush (because its easy to get through), or review your notes.