This is entirely dependent on your school. Most of MS4 will be elective and away rotations at all schools, but whether or not you have required rotations is school-dependent. I wouldn't waste all those electives doing things in your field though. Do a couple, but unless you're going into a very competitive specialty and need months of aways, concentrate on taking electives you'll never have the opportunity to take again and/or things you should know going into your field.