I personally took Medicine in my fourth block.
Since Radiology is pretty competitive, it becomes quite important to honor as many 3rd year clerkships as possible. Honors in Medicine and Surgery are particularly important. That being said, I think it would be better for you to "warm up" with something else. I personally started out with OB, then FM. I followed that up with Surgery and then later Medicine. You can hone your presentation skills, your differential diagnosis skills and get comfortable with getting things done in the hospital (getting path slides read, talking with the social worker, et cetera) and comfortable in the OR ( via OBGYN).
Being helpful, enthusiastic and having a good attitude goes a long way when coming to 3rd year clerkship evaluations. It's pretty hard to be helpful during your Medicine or Surgery clerkship if you don't know up from down in the hospital.
My 2 cents.