Depends on the fitness of Nadal.
Nadal has the number of all the other players. Sure, Djokovic went on a hot streak and damaged Nadal's confidence, but he put that right within a year, nearly beat him at the Australian, then went on to whip him 3 times in a row.
If Nadal can stay healthy for the next 4 years, then I think he can win at least 6 more slams, which would put him at 17. And you can't count him out because of injury. His career has looked "over" several times, yet he bounced back to have some of the best seasons of his career. The fact that they don't feel his injury is serious enough to require surgery leads me to think it'll be another one of those injuries where he comes back playing better than ever.
Federer has 1 more in him, so I guess he finishes on 18.
Djokovic will probably finish on 8.
Murray will finish on 4-5.