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.
I don't know what cave you have been living under but my merits are displayed right there! Minimum required merits to become a legendary are 1000 and if you look at my merits, you shall see I'm pass that.
When people tell you,"Post in meta dude.. You'll get merits easily..." And then you come here saying crap without knowing anything. Nice.
Nailed it.