Wait - let me get this straight? You want to apply to med school next summer, the summer before your senior year - right? So you will be applying to med schools in the same time frame (summer prior to senior year) as all other traditional applicants?
So if I were you, I would apply to schools without stating that you don't intend to complete your senior year (don't want to add any hurdles to your app) and if you get acceptances, only then should you contact those schools to see if this would pose a problem for you prior to matriculation at that school. If it's a problem for them - then complete your senior year, if not, enjoy your time off. This way all your bases are covered and you won't have to possibly reapply if schools decide to toss your app after seeing your note.
P.S. I would revise your reasoning for why you don't intend to complete your coursework. The "my fourth year is specific to my major and has nothing to do with med school bit is not going to fly as it applies to practically all non-bio or non chem majors. If anything, your major is probably closer to medicine the most of the social science majors such as those in history or religious studies.