Well, there are many different things that contribute to your rank. Mostly challenges but also, gaps and Secret Spots. Here are my rough stats when I hit This Rank:
With half the gaps and all Secret Spots.
It seems that when you have most challenges done and less than 20% AM it will Unlock!
Just do as many challenges as you can and keep increasing your rank. When at Rank 8, it will take a while to get to this, but be patient. You will also "Skip" rank 5. Because once you hit it, you will be able to go to Rank 4 almost right away.