One of my general surgery attendings told me I should take as many medicine electives as possible b/c I'd "have the rest of my life to learn surgery".
I really didn't take his advice b/c #1: I really can't stand medicine's style and didn't want to spend my last year of freedom miserable and #2: although the concepts are the same for, say GI and Gen Surg, the two specialties take different approaches.
I really think you should turn your 4th year into a vacation (at least the spring semester). Radiology is definitely a must b/c it's easy and you'll learn a ton of practical info. A month of the ICU at the beginning of the year is not a bad idea either. Many of my friends did the medicine thing and took Cardiology, GI, Pulmonology, Nephrology, etc. all of which should help you immensely in ER or GS.
If it's not mandatory at your school like it was mine, I HIGHLY recommend taking a rural rotation. I learned more and had more independence in the rural area where I was pretty much the ER "doc". I triaged a lot of stuff, took care of what I could, and always communicated with the attendings who were usually in clinic but would come to the ER for every patient even if it was just to sign my note and say 'hey' to the patient. It's just nice to be put on the spot where the nurse/patient ask you what you want to do.
I do regret not taking at least of those medicine electives simply b/c I'm so much dumber right now prior to beginning PGY-1 <img border="0" title="" alt="[Eek!]" src="eek.gif" /> . BUT, I had a blast during 4th year and definitely wouldn't trade it.