Once the last mission of Sequence 3 - Freedom of the Press is completed cutscene will begin where you will be introduced to Ned Wynert who will be assisting us with the finest transits. Now Evie will start her exploration for the Piece of Eden whereas Jacob is eager to hunt down the Templars and Starrick from London.
You will now be able to look at the Assassination Wall on the train which will guide you with Starrick's associates to be taken down and here we start with the Sequence 4: A Quick and Reliable Remedy part A Spoonful of Syrup.
To Begin with the first part of Sequence 4 where you need to investigate a new drug that is ravaging London to uncover its creator. Head to the Docks to start with the mission and first you need to tackle the merchant. He will provide you the information about the thug who has the distributor's location.
Now you need to steal the information from the Thug, but it also has a side objective to do it without getting detected. Once you have gathered the Information head straight to the distributor's Boss and Kidnap him. Try to remain undetected to complete the side objective. Interrogate him to know the Syrup's origin and Hence completing the Mission as well.