I did three PM&R rotations--I think that was one too many.
I'd highly recommend rotations in neurology and palliative care, unless you can do them as an intern. I did neuro as an M3 (will also be my last rotation of intern year) and palliative care as an intern and I thought both were exceptionally strong rotations (dependent on your program of course.)
I agree it's worth doing stuff that you won't get to see again--do some things that you feel are interesting. Ideally, do a few rotations that prepare you for intern year (two sub-internships, including an ICU month), a few rotations that are relevant for internship & PM&R (neuro, palliative care, cardiology, rheum), and a few light and/or exciting rotations (radiology, wilderness medicine, etc...).
At my program we had "light" rotations on mastering EKGs (cardiographics) and a "prep for medicine internship." Both were entirely lecture based and generally about 4hrs of lecture max, so they left quite a bit of free time. But they were incredibly helpful and high-yield. Seek out current 4th years at your school (current interns/residents from your school would be even better) and ask them what they thought the best 4th year rotations were.
EM is also a good rotation to do--it really gets you thinking different. I do think it was much better as a resident though. (It was required both as an M4 and in my PGY1 year--otherwise I would have just done one rotation).