An easy way to do this one is to make your way to the viewpoint first and kill the sniper in the tower just opposite. Exit the restricted area and run all the way south until you reach the water. Swim into the restricted area and follow the coast to the east (using stealth swim if necessary) until you reach another sniper tower. Climb to the top of this and deal with the second sniper here.
Return to the base of the tower and sneak along in the stalking zone between the nearby warehouse and the water. Through the open door here, if you get the right angle you should be able to spot the poison reserve barrel that we need to destroy tucked away in a corner. Shoot this to have the guards fall asleep.
Return to the previous sniper tower and again use the water to swim around until you spot the third sniper tower (again using your stealth swim if necessary). Use the rooftops, stalking zones and avoid patrols as you work your way up to the tower. Kill the final sniper.
Finally, head over to the green search zone nearby and use the rooftops to get a nice view of the area. Activate your Eagle vision and be on the lookout for a group of three patrolling enemies, once you spot them, your eagle vision should identify the target. Now either jump down and attack the target or stalk him along the rooftops and perform an opportune air assassination.
Once all of the four targets are pushing up daisies a scene will play to indicate successfully completing the settlement takeover.