In The Last Stand: Union City, the player must break through the wall to Union Island in order to escape. Jack tasks them with finding C4 in order to blow up the wall. He suggests searching the abandoned military base in Waterside.
The C4 is split into 3 components: 1 Timer, 3 Blasting Caps, and 3 Blocks of C4. The parts are scattered throughout the base, presumably as a safety precaution. The Timer is located in the middle building, along with the key to the base's gun cage, which is where the C4 Blocks are stored. One of the Blasting Caps in also in a nearby footlocker, while the other two are located in the rightmost building.
Once the player has retrieved all the components, Jack uses it to blow up the wall.
- ↑ http://books.google.ca/books?id=Yfs8ewaKEZYC&pg=PA40&lpg=PA40&dq=c4+is+a+clay+like+substance&source=bl&ots=zB_tlHcEet&sig=Ziomg6XZOQBb0gjmvH8vBhOZ-Rw&hl=en&sa=X&ei=mHAZVIPoK9GWgwTnjYLYDg&ved=0CCsQ6AEwAg#v=onepage&q=c4%20is%20a%20clay%20like%20substance&f=false
- ↑ http://science.howstuffworks.com/c-42.htm