Mission 04: Backseat Driver
Approach the start point of the mission and trigger it. When you do, the mission marker will turn purple and lock. This means your game has been invaded by a fixer and they are trying to steal Aiden's data. Fat chance. Hop in a car and burn rubber to the new objective marker nearby. The name of the fixer will appear on the screen and when you approach the next objective marker, the game will give you a quick bit of text explaining what invasions are.
Essentially, you have a set amount of time to walk around a small area and profile people in it until you find the fixer or run out of time. At set intervals, the search area will shrink, making it easier to locate the fixer. A couple of things to keep in mind: Look inside cars and watch for any fast movement (people who have played Assassins Creed's multiplayer should feel right at home) and remember to look above and below the specified location. You should have plenty of time to locate the fixer and once you do, you need to kill him or take him down (try not to alert the police when you take him out).
Once the fixer is on the floor, the game will inform you about side/online missions once more. Exit out of the menu and make sure you have some jam coms items (craft some or head to a store to buy some components). The upcoming mission involves a police chase, so make sure you have some of these useful items as they will make like far easier.
Objective: Reach the west island for the driving contract
Once you are back at the parking lot and have started the mission, jump in one of the nearby cars and take a little drive to the new objective marker. As Aiden approaches the area, he will enter the area being scanned by police.
Objective: Reach the fugitive undetected
Apparently they are already after your passenger and you have to reach him without being detected. Get to the new marker as quickly as you can while staying out of the scanning circles on the map as much as possible. Remember to use a jam coms item if your detection percentage goes over 85% to avoid being picked up by the police scanner. When you reach the second marker, you should meet your passenger.
Objective: Escape the lockdown area undetected
After the passenger is in the car, it'll be up to Aiden to get him out of there. You need to exit the police search circle without being spotted by police cars or helicopters. I would advise you to head along the back alleys to the south of your starting position, sneak through the courtyard next to the nearby station and then head south to the objective marker, using the alleys to hide when needed. Watch the mission video for a better idea of where to go (but that's the general idea).
After hitting the first objective marker (the edge of the search circle), the pursuit will die down and you can quite easily drive to your next destination. Watch the cut scene when you arrive and the mission will end.