I completed an MSTP program several years ago, and although I didn't do my MS3 before my PhD I will comment because I feel strongly about this. I think it is a bad idea on multiple levels. In fact, I cannot believe that your program even allows it. It seems like it would encourage people to bail out of their PhD.
First of all, the first two years of medical school are minimally relevant to your practice of medicine, so it doesn't really matter that you've forgotten most of it when you go back to the wards for your MS3. You can rapidly pick up the clinical skills that you need when you get back, and the little knowledge you need you can learn quickly However, when you become an intern, you want your practical medical knowledge (all from MS3) to be as close as possible. There is often not going to be anyone to tell you what to do.
You might say, well, you can brush up on your skills during your MS4. At most schools, MS4 is a joke that is basically designed to let you take leisurely electives during interviews.
Second, most of the knowledge needed for step 2 and step 3 is from your MS3. A large gap between them, particularly if you do a specialty (anesthesia, rads, derm), is only going to make step 3 more of a pain. Let's face it, your 6 year old knowledge of OB isn't getting any better.
Third, being that close to the end of the MD is going to make it all the more painful to finish your PhD for 4 long years when you know you have only 4 clinical months left and could just bail.
Finally, I might just ask what your actual motivation is, because it doesn't really change the timing. Moving your MS3 to before your PhD doesn't change the fact that there is going to be a critical time of the year after which you must finish or wait another year. It's just going to shift this deadline.
IMO, bad idea. Maybe someone else can offer you more constructive advice.
