depends totally on personality if the first two years are better or the second two years...
if you liked having your own schedule and not having to show up for anything unless you really wanted to and not stressing about "competing" with your peers for grades, but rather just competing with the material itself...then the first two years are better.
it definitely is much more structured, you have to be there many many longer hours, and you don't get to choose when you leave (most of the time). On the other hand, much of the time commitment is not about actively using your brain (although it helps), but rather using your hands, pen, stethoscope, mouth, ears, and eyes. The thing that I found very hard was having to worry about being evaluated subjectively and realizing that being memorable in one sitting for 30 minutes with the attending who grades you on 1/4 of your grade can count as much if not more than busting your butt 12 hours a day for your resident/intern. You have to figure out how to play the game a bit if you really are worried about doing well.
For the most part though, you do well by just showing up all the time, on time and prepared and being pleasant to be around. That counts for a big chunk of 3rd and 4th year and its surprising how many people can't do that for various personality or other reasons.
It is a totally different world, and the biggest thing is the amount of free time that you take for granted as a 1st and 2nd year that you realize was a total blessing. 3rd year is definitely the year that requires the most stamina, but it goes by fast because you're doing something different every few weeks. 4th year though, aside from a few very intense rotations with a lot of responsibility ends up being a pretty relaxed year (except for the stress of getting into residency).
The material, in the clinical years, is a thousand times more interesting.