Four Stores can be a very quick and easy job, or quite challenging, depending on instancing. Your goal here is to steal a set amount of money from within a safe that spawns in one of four places: a convenience mart, a computer store, a fine china shop, or a restaurant.
Casing all four buildings is of the utmost importance here - there are four possible spawn points for the safe. The restaurant and china shop are the easiest to work with, while the convenience stop and computer store are more challenging.
The china shop is the easiest place to defend if the safe spawns there - the break room in the back of the shop has only one way in. Should a shootout occur, this is a wise place to take position; a Doctor Bag and an Ammo Bag will help as well. If you take the stealthy approach, be sure to take care of the cameras and any civilians inside. Also, be careful of using the rear entrance. It's easier to set off a camera alarm from there.
If the safe spawns in the restaurant, things won't be too difficult either. The kitchen in the back of the store makes for a good holding point, as police must either round a corner or hop through the window - this is a big advantage. Throw down a Doctor Bag and an Ammo Bag, and you'll be good to go. For the stealthy approach, watch for cameras in the kitchen. You'll have to pop them as you round the corners.
Should the safe spawn in the convenience store, your main concerns will be the clerk and the security camera. If a police assault occurs, it's best to use the counter for cover. Don't neglect the back office, however - the cops can enter through there too. You should also think about throwing down a Doctor Bag and an Ammo Bag to fortify your position.
If the safe spawns in the computer store, you're in for a tricky time. Like usual, cameras and guards will need to be taken out. The challenge comes with a police response. The computer store is nothing but glass, so cops can enter from pretty much anywhere. Use the counter as cover as best you can, and you ought to make it through. Don't forget a Doctor Bag and Ammo Bag either.