http://en.wikipedia.org/wiki/Sean_Combs
P Diddy ran a marathon recently. He had he own personal trainer. Ran 4:18 and was very please with his time.
Under 4 hours is not realistic. Very few first time marathoners succeed in running the entire 26 miles. You will be walking part of the time. To break 4 hours you will need to average 8:30/mile or 5:25/km. Run a 10k. If you can't break 50 minutes, forget 4 hours. It's just not possible.