FF--my department offers many 2/4 credit courses. Usually what happens is M.Arch./M.L.A. students take the 2 credit option, while the Ph.D. students take the 4 credit option. Everyone does the same amount of reading, attends the same number of courses, does the same amount of base prep for class, but the 4 credit students write more.
I've never heard of this practice! At my school, the often cross list courses as 4xx/5xx and you only get grad credit if you take is as a 5xx and essentially they make you do the same (write an extra major paper, do a big presentation, etc.). But that's a little different I guess since it is differentiating between undergrads and grad students as opposed to masters and PhD students...