The fourth should be connected to an industry of some kind, so there's an obvious, visible purpose to the cars there. Empty hoppers in, loaded hoppers out. Sometimes a boxcar with supplies arrives. Flatcars with logs in, boxcars with finished products out. Petroleum terminal or chemical industry.
Tank cars out and in.
Not much, really. You'll need at least three types of freight cars, but except for one type, you'll need only one or two examples of each. One loco, two passenger coaches, seven freight cars of three types and a small layout like this is almost at capacity. If you think it's too little variety, have them exchanged via the interchange. Much of the inspiration for these recommendations come from articles by Hans Svensson in the magazine Allt om Hobby .