In Quake 4, Slugs are solid depleted uranium slugs, most likely lined with a magnetic metal like cobalt so that the magnetic coils can fling them through the air at supersonic speed. They are used as ammunition for the Railgun.
In Single Player, Slugs come in triangular 6-round packs, but only gives the player 5 rails; whereas in Multiplayer, Slugs come in 10-round green boxes.
Single Player
Multiplayer