#4 is located at the northest point of the map. From Paleto Bay (the town on the north west coast), head east along the main road. Keep going until the map shows a road that has a little loop in it.
Just a little down from that, is a hidden trailer park. If you zoom in on the map, you can see the grey road coming off the main carriage way. It looks like a snakes tongue.
Here's an image to help.