1500 XP, Pistol
After completing The First Head of Cerberus, talk to Garrett again to start this one.
When you get to the first building on the map leads you to, you should see a ladder out front. Climb up and head right where you'll have to jump between platforms
For the second one, follow the mini-map to the next building. The ladder is on the back side facing the water. Once up top head left, until you see a caged area, you can get in there from the platforms on the edge of the building.
For the third one, continue along the water until you come to a staircase, go up it. To the right is another staircase that is blocked by crates, move the crates to get to the speaker controls.
The fourth one should be in plain sight as you follow the map except it's up on a fence. Go along the fence until you come to a spot where you can climb up.
Return to Garrett to get your reward. Talk to Garrett again to start The Third Head of Cerberus quest.