i highly doubted you could finish DMD and PhD within four years. The shortest time is 7yrs, that assumes everything is working as the way you want. Research is tricky sometime, you can't be absolutely sure about the results and this is why sometimes it can take longer than you expect. Are you sure, u like to do research or at least what to expect. Well, i know i'm not answering your question, i am not aware of the combine degree program at UPenn. I know they have oral biology related graduate program, but my understanding is _it is a separate program from DMD. Of course, a student can do both, but UPenn doesn't offer combine degree as in other schools which already have established combine degree program and support student financially. But i could be wrong, they might add this new program recently. Anyway, good luck!