to be honest, M1 is usually considered a "better" year.
M2 holds a lot more information to memorize, but you might find eat easier to tolerate bc it's more clinically (medically?) relevant.
M3 has less information to memorize, although you still will probably have shelf exams to study for each clerkship and have assignments from your attendings; the emphasis is on your clinical work though, and it's usually the hours (not the information load) that has people complaining.
M4 is the best year for most. the expectations are lower, the hours are better, you usually don't have shelf exams and aren't expected to study as much. if rotations have nothing to do with the residency you're applying for, the expectations are sometimes even lower. it's a year of taking your steps, applying, and travelling for interviews.
basically, the memorization backs off in MS3 year but the 2nd and 3rd years are probably the toughest overall.