That's a question I still ask myself, since I've considered getting an MPH myself. I think moneywise it won't really benefit you, necessarily (I'm assuming they are charging you some tuition for the MPH?).
If you want a career in public health, such as working for the CDC, then yes I think it would be useful.
If you just want it as a feather in your cap, it might help for getting certain residencies like peds or IM, but if you do all right in medical school you probably wouldn't need any extra stuff like an MPH to get into a higher tier medicine or peds residency. I don't see an MPH being very useful for someone like a dermatologist, radiologist or most surgeons.
For just residency matching, having done research with some famous attending, or just one who is well connected (i.e. knows folks @other residency programs and can help get you in) would help more than just having an MPH.
I would get the MPH only if you are really interested in public health and think it will be a big part of your career.
I'm also wondering how they cram an MPH and medical school all into 4 years...I think you'd have little or no elective time during 4th year, which could potentially suck, because that's when you get to do all the cool, fun clinical rotations that you picked out yourself (electives!!!). Just asking/thinking...