Disrupt the Greengill Coast opens up as soon as Phase 4 begins, and is given by Captain Valindria. She can be found on board the ship(known as Silvermoon's Pride) parked in the Harbor which you took in Phase 3.
Captain Valindria is apparently the SSO operative in charge of dealing with the Naga infestation on the southeastern section of Quel'Danas. Like most people in Azeroth, she needs our help. This isn't just another kill quest, but it is a kill quest in disguise. I recommend doing it alongside Don't Stop Now... because you'll be killing the required mobs simultaneously.
It turns out the Darkspine Sirens are mind controlling the Murloc slaves you see throughout their camps, and they're doing it using some fancy orbs. An Orb of Murloc Control is what you want, and they drop off of the aforementioned Sirens. You want to throw and smash them into a camp of murlocs to release them from their control, and turn them on their Naga masters.
What these means is the Orbs work like grenades, targeted AOE spells, et cetera. When you right click the item, you'll get a targeting circle that the previous bombing quests have trained you to use, and throw them at a pack of murlocs. You need to free a total of 10 murlocs, so try to get as many of them in the ring as possible to cut down on Orb hunting.
Simple quest, and fantastic to do alongside Don't Stop Now... as I mentioned before. Doing them together will net you some easy money. Even if you don't do the other quest with this one, you'll still get a pretty decent reward. 11 gold, 99 silver, and 250 Shattered Sun Offensive reputation. Note that this quest doesn't help to progress anything on the island, and will not change once the Alchemy Lab is acquired and the Monument to the Fallen is completed.
If you need more information on this quest, please head on over to Wowwiki or Wowhead!