I would definitely not push third year rotations into fourth year.
The ERAS stuff, as I recall, has to be in by November 1 - this means letters of recommendation, dean's letter, etc, both of which are somewhat contigent on doing your sub-I's. Most people that I know spent the first couple months of fourth year doing sub-I's and away rotations to finalize their decision of what specialty to go into and also to get good experience at their own school, other schools, and line up letters of recommendation.
Everything is pushed ahead a month or so for the SFMatch. You basically have the first three months of fourth year (July, August, September), at the most, to include on your application and in which to get letters of recommendation. So, if this time is taken up with finishing third year rotations, you may be out of luck.
As for studying for the boards, my school gave us about 6 weeks off between second and third year. I took a four week Kaplan class, then studied on my own for about 10-12 days, then took about 10 days off for vacation. I had done a bit of preparation at the end of second year, but no serious studying until Kaplan started. If your school offers board review stuff during second year, I would go. I don't know what your curriculum is like, but we did path/pathophys second term of second year and many people used each organ system as an opportunity to review normal anatomy/physiology as well as to learn path/pathophys.
As others have said, studying for step I ought to be a review. You know yourself better than any of us know you, but I think many people have trouble studying intensely for more than 4-5 weeks (or less). So, again, I would definitely recommend starting third year on time, especially if not doing so means giving up your first few months of fourth year electives.
I would also speak with deans at your own school, if you haven't done so already.
