As a soon-to-be MS4 my anesthesiology faculty advisor recommended 2 months of ICU and 1 month each of cardiology and pulmonology. In addition to that I signed up for an ER month and a clinical pharm course for flexibility around interview time (dicactics/online only). The only anesthesia-related elective I'm taking is OB anesthesia, which is a popular rotation at my school for enthusiastic attendings, lots of procedures, and good hours. I got to do a month of general anesthesia 3rd year.
In the words of my advisor, "You'll get the best long term return-on-investment of your MS4 tuition if you spend your time learning not how to be a good anesthesiology resident, but a good intern."