How to find the legendary ships? [AC: Rogue]

i was flipping the google upside down but i failed..

all i see was "it will appear on the map after sequence 3" and i already finished sequence 4 and im looking for it..
i was thinking if its like blackflag that if i go to the four corners of the world, i can find it but this time its not..

so where can i find them? and how many legendary ships?