This settlement is another fairly easy one. We need to kill three snipers and a guard captain and three of these four enemies are within spitting distance of the lighthouse with the viewpoint on it. The ledge below the lighthouse is outside of the restricted area and essentially gives us access to two snipers overlooking the restricted area below. Take your time to eliminate the pair of them and look down into the red zone.
You should see a group of three guards patrolling about, use your eagle vision to identify the captain of the three and then use some of the ropes strung around above the restricted area to perform an air assassination on him when he walks below. Kill any guards that attack you at this point.
The final sniper can be found in a guard tower overlooking the docking area. He is by himself and inside his own, smaller restricted area. There is one other guard here you can quickly dispatch and then once he’s gone, you can pretty much walk up and shank the final sniper with little or no resistance. Once all four targets are dead, the settlement will be yours!