Third and fourth year are night and day. Fourth year is very reasonable hours (often 9-5 with the opportunity to schedule in nice vacation time). The only catches in 4th year are that you have to apply and interview for residencies, which is an ordeal, you will have to do sub-Is and away rotations which can have more intense hours for a month or two, and you have to take the two parts of Step 2.
Third year, by contrast, will be your hardest year of medical school. Most people find it "better" because you finally get out of the classroom and onto the wards, but the hours in some rotations will be rough. in-patient medicine, in-patient OB/GYN and surgery all tend to have the longest hours. Being on-call, meaning you don't get to leave the wards for 30 hours, can occur as frequently as every 3rd or 4th night on some portions of these rotations. In surgery, it's not uncommon to have to be in the hospital before 5am (it's a very morning oriented field). And not getting days off on the weekends during parts of these rotations is not uncommon. The biggest issue tends to be that your time is not your own during third year. You really can't schedule things. Also bear in mind that there tends to be a shelf exam at the end of each rotation, so you are expected to be studying for an hour or two each day after your 13 hours on the wards. On some rotations there will be down time during the day to study, but others not. And you will frequently be pimped on your lack of knowledge and have to spend time looking up things you don't get right. And you will have presentations during some rotations that you have to prepare for. So yeah, it's rough. An ordeal to get through. you will also have your share of outpatient things, and the less hour intense (more 8-5) rotations like family medicine and psych, to break up the intensity.
You will probably get the major federal holidays off (memorial day, labor day, thanksgiving) and will likely get a winter break (xmas to new years). Usually not much of a summer break after third year -- maybe a week before 4th year starts.