When you find all four vehicle keys you get the No Down Payments achievement. Also because some keys are in different paths in the game (paths you can't turn back on), you will need to play through at least twice to get all the keys! Anyway here's the guide:
Once you're done helping Mia out at the theatre go to the restaurant where Scout was.
Scout will have left a letter and soem keys to a vehicle on the kitchen table.
2. Lemon Holl
When looking for fireworks you'll come across a very dark room on the rooftops and a ladder going down to a yellow bus.
Right before you get on the ladder, look for a bedside cabinet. The keys are on it.
3. Archer Creek
Snipers will cover you during a portion of Archer Creek. When this happens, you should see a building titled "Archer Creek Utility".
Search through all the side rooms that have wooden doors to find the key. It is on a shelf.
This key can only be found on your second play through.
Once you've pushed the car in order to jump over the fence you'll need to proceed through an extremely dark shop.
At the end of the store, a Walker will surprise you by smashing through a wooden door.
The key is in a side room off of the room the Walker came through.
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