My account being a perfect example here. I had 1000 default merit and I think activity as same as the OP but I did not reach the legendary rank
So, if The Pharmacist is right which is HE is right, in the next 4 weeks or less you are going to celebrate your Legendary rank!!! Congratulations in advance mate. 
That is if he gets the required number of merits for the new rank, his activity might be due for the new rank in next 4 weeks or less just like you said but he also needs the merit to move on to the legendary rank.