Four Stores is exactly what it sounds like. Four Stores lined down a strip of road, with some safes and such in each one. You need to get a certain amount of cash before the van starts coming.
Your fastest ticket to getting cash however are in the two ATMs in the map. They can be cracked open by even the basic ECM, and depending on what difficulty you're playing on, can give the min money needed in just one.
If you're interested in getting more cash though, head around the four stores and start drilling any safes you find.
There are also some cash registers scattered around which are opened instantly and have some small cash in them.
If you were lucky enough to have a big safe spawn, you can find lots of instant cash and even a few Bags of cash!
As always, once the truck arrives, throw any bag items into the back and step into the evac zone.