No matter what you do as an MS3/4, you really have no significant impact on the medical aspects of patient care. You'll feel like you do, and that you're going to screw up and the patient will be affected.
Agreed. Unless you're actively messing with things you shouldn't be around (pulling out lines, etc), you're not going to impact the medical care, but as the person with the most time on the team, it's very nice when you can just talk to the patient. Doesn't necessarily have to be about their diagnosis, though you can certainly do your best to answer their questions, but just treating them like human beings. Go play with your peds patients. Reassure parents (just remember... you are on their side, both trying to make sure their kid is getting better). Talk to your adult patients. Try to learn about them and how their disease impacts their life. When I acted as AI, I had one patient that was there a good chunk of my time on service, and I'd just check up on her during the day, talk to her family... they appreciated that as much as the actual medical care that she was getting, and even when I went off service, I went to check in on her, and her family still looked to me with their questions. You can make a huge impact by just talking.
As far as studying goes... ask what people at your school use. There are a variety of resources available for each clerkship. For FM, the AFP has questions available if you sign up as a student member. There's Pretest if you like questions (MKSAP for Medicine), Case files if you prefer cases, and Blueprints if you like a more text-book format. When you get a new patient, read up on what's going on with them. Figure out what the differential might be based on what's going on. What labs you might've wanted early on to narrow the diagnosis. What the labs tell you. Up-to-date is a good resource, but there are others, and don't be afraid of review articles. Peds in Review has a lot of really good review articles that go through diagnoses pertinent to peds.
Step 2 focuses on diagnosis and first-steps when you have a complaint of something. The good news is that you do this during your clerkship, and you take a mini-Step 2 each time you take a shelf, so by the time you finish the year, you should be a pro at it. That's why the national average for Step 2 jumps up about 10 points.