MS3 varies by rotation. Some are notorious for long hours and waking up before the crack of dawn, such as OB/GYN and Surgery. 3rd year has for the most part, the same core rotations(Surgery, IM, OB/GYN, Peds, Psych, FM, Neurology) with shelf exams. The workload varies by rotation. Most of the cores have an inpatient portion as well as an outpatient portion. Clinic portions have easier hours, and you usually see patients before the attending/residents, present to them, and get feedback. The ward portions expect the med student to pre-round on patients they are following, then formal rounds, and doing admissions/going to OR/doing consults/assisting deliveries. Med students usually take call which can simply mean staying later, or staying overnight with the next day off. Also, weekends on occasion. Shelf exams are at the end of the rotation, and with daily reading, are manageable. Some can seem daunting(IM with the broad spectrum, OB with the bundles of new information) but if you start reading from the first day, it'll be all good.
MS4 is more or less the "semi vacation and easier than years 1 and 2". For the most part, it's electives with easier hours, no exams, no calls, and more laid back. Near the beginning, people like to do sub-intern rotations which can be brutal, but at the same time, a key way to get a strong LOR. There were several weeknights where I was able to go to happy hours after getting off work as an MS4 😛