UHS will try to blow the "choose a small hospital" up your butt for 3rd year rotations...and I believe this is all political on their part. They certainly do NOT have the strongest 3rd year rotations, so it is necessary for you to research your choices well for core rotations. 4th year is entirely up to you, and most choose to either go back to where they consider "home" or they do alot of traveling from site to site. From what I can tell, most of my friends spent at least 1/2 of their 4th year at university (allopathic) programs.
UHS has more control over 1st and 2nd years, and since I left, they have changed the curiculum. So I cannot speak for that. When i attended, the 2nd year was very strong and prepared us as well as any program, be it allopathic or osteopathic.
UHS is a quality program during the 1st and 2nd years, they have little to do with 3rd or 4th years...that is primarily the responsibility of your core hospital rotation site.