I agree with the above--most attending will know you're doing your first rotation in rehab and not expect you to know much (especially on the MSK side of things). But you should definitely know how to do a comprehensive near exam--that's something that you will be expected to have learned already (but not necessarily be perfect at).
I'd highly recommend doing a neuro rotation at some point in medical school--after a medicine Sub-I (which is usually required), it'd be the rotation I recommend the most (assuming it's a good rotation). Ortho, neurosurgery, and sports could also be really helpful/interesting. So will things like cardiology, palliative care, rheum. I wish I'd done a rotation in ID.
Unfortunately you only have so many electives you can take--make sure you also rotate through things that will help you for intern year. I'd try and balance having more fun electives (like sports) that may not help you much/at all intern year, with ones that will help a lot (cards, ID, neuro, palliative care, critical care--all rotations that will help for inpatient rehab as well). Don't forget to do a lazy elective or two (rads)--it is 4th year and you should have some time off/light rotations in there! (Especially during interview season).
And your most important rotation: vacation. Make sure to take a fun one before you start intern year if possible. I went camping for 5 weeks--best trip of my life.