I think that in the 4 year degree, a student at a US university does as much in depth in their subject as a student at an equivalent UK university does in 3 years. I took more classes in IR than my major actually required, as did most of my friends on my course. Depending on how you structure your time, you can actually do significantly more than a British student could by taking more units or doing directed readings (individual study one on one with a professor).