Few schools offer a 4 yr combined MD/MPH - USC, U Miami are only 2 that I know of off the top of my head. In my case my school would only pay for the MPH tuition if I did it at USC, and I don't think it matters too much what school you go to (unless it's John's Hopkins, Harvard, etc.). There very few circumstances that people on this board would recommend you obtain an MPH, three common reasons being 1) you are interested in public health, 2) you are interested in academic medicine AND plan on doing research during the year in addition to the coursework, 3) if you need take a year off of med school to couples match (not commonly said on this board, but there are 3 MD/MPH students in my cohort doing the MPH for this reason). Most would say do the MPH after your 3rd year.
My reason for MPH is #3 on the list. I'm happy I'm doing it but would have preferred to not take a year off med school and don't see it adding too much value to my residency application / career.