Story related and cannot be missed.
There are 6 memories in DNA Sequence 4, 5 of which have optional objectives which completing them during your initial run through will help you with another achievement later on.
Memory 1 - The Prince's Banquet
- Beat Yusuf to Topkapi Palace
Memory 2 - An Uneasy Meeting
- Do not be detected
Memory 3 - The Fourth Part of the World
- Get in and out of the port without being detected
Memory 4 - Signs and Symbols, Part 1
- No optional objective
Memory 5 - Galata Tower
- Complete the level within 6 minutes
Memory 6 - The Mentor's Wake
- Do not lose more than 3 synchronization points
Once you complete memory 6, the achievement should unlock.