Really? He needs 4 more. Will he be YE no.1 in 2013, 2014, 2015, 2016? I doubt it. He will be 26 next year i.e. end of 2016 he'd have to be 30 and the no.1 player. His game isn't like Federer with lots of cheap points - he declined a bit in 2012 vs 2011 and no reason to think he'll be better at age 28 than he is now. Don't think that figure is reachable nor do I see him being this committed for 4 more years.
when he's 30 years old, the competition will not be as tough as when Fed is 30 years old, that's a big difference, people look at Djokovic against today's competition to guess how it will be hard for him in 4 years but it will probably not be as hard.
Besides, this kind of game can last long, see Connors and Agassi. There are more players like that among players who lasted long than players who had many cheap points. He's complete, has no weakness, a light body, a very good backhand, that's perfect to last long.
If Nadal ends up with 13 hypothetically, Novak will need 9 more slams to overtake him! 9! That's as many as Lendl in a career!
Overall I see him ending up in a Lendl like position - LOT of consistency, lot of finals, long time at no.1, but fewer slams than his top rivals (Fedal) and a rung below them in the all-time great stakes. Nadal, like Borg, will have historically a short time as no.1 relative to his stature but in the context of him being no.2 to Federer for so long that's understandable
Lendl really started winning slams when he was 25 years old, Djokovic can do the same and it's easier now to win Wimbledon than in Lendl's time.
Besides, when Lendl got old there were Becker, Edberg, Agassi, Sampras, Courier and before that Wilander ... (all players who were already very impressive and won a lot at a very young age)
What seems the main opposition to Djokovic in next years is Murray, and Nadal at least on clay, elsewhere we will see.
We will see what they can do but imo Djokovic could win many slams, Murray is not as good and Nadal is fragile. Lendl never won more than 2 ones in a year.