Forgotten is an achievement/trophy in Metro: Last Light is awarded to players who find 4 hidden ammo stashes on Quarantine. There are actually five clips of Military-Grade Rounds, but only four are needed. They are all found in close vicinity of each other, in the abandoned part of the station. Once you get through quarantine and are let into the station, you will enter the area. The door will shut behind you. You know you are in the right area if you see a hidden Stalker with Night Vision Goggles on. Four of the five clips are found behind partially boarded-up train car windows (the first one being immediately right of the door that seals behind you, before going down stairs) and one clip is on a white sink soon after you go down stairs.
Below is a video guide if you are having trouble finding them all.