OK...So maybe you don't understand how rankings work.
If Murray wins the AO title, he'll add 1280 points to his total, because last year he earned 720 by virtue of his semifinal appearance. So he drops 720, gains 2000 for a net gain of 1280. Let's assume he defends his Brisbane title. So Murray would have, after AO 8000+1280=9280 points.
Now let's assume that Nole loses in the first round, better still, doesn't play the AO for whatever reason. Then he'll lose the 2000 points he won last year. Then he would have 12920-2000=10920 points.
So even in this situation, with Murray winning Brisbane and AO and Nole not playing at all, Murray would have 9280 points and Nole 10920 points at the end.
Not enough for Nole to lose his #1.
The thing is, Nole earned 4920 points more than Andy in 2012. It's a huge lead. Andy could pass Nole in 2013, of course, but it will take time. 4920 points is a lot of points to overcome.
Specially since Nole is not likely to lose before semifinals of any important tournament. His consistency and excellence secure his no1 ranking as he has been making semis or better of almost every tournament he's played in. Talk about an achievement. None of the other 3 are close in that regard so his lead is safe for a long time bar some disaster.