I actually think every medical school will let you go abroad sometime in your 4th year if you really want it. Therein lies the catch. Some schools will have official programs and sister sites in other countries so getting academic credit, setting up the rotation, and coordinating with the hospital in question are all easier tasks to accomplish. If a school doesn't have an official program, then the majority of the work (and paperwork) needed to get such a rotation to happen falls squarely on the shoulders of the student. There's also no guarantee that you'd be able to receive academic credit for the time you spend abroad so, if you really want the international experience alone, you can set it up so that all your free time 4th year is consolidated into a hefty chunk of time that can be spent in the country of your choice. I think it really depends on the amount of desire to accomplish this particular goal in the student. If you want it to happen, you'll find a way to make it happen.
(This is at least what I've gleaned from the answers to my questions about international rotations at various med schools.)