The first question is whether you really need four separate serving pressures or not. Lots of people will want to accomodate two pressures - one high pressure (30PSI-ish) for soda or force carbing, another for regular beers, or they'll want one for serving something like an English bitter at a lower carb level.
Usually, though, people get by with maybe two different serving pressures.
Now, to DO this, you need multiple regulators. You'll need a primary regulator that's connected directly to the tank, and three secondary regulators that will each step down the pressure. You aren't going to find a single regulator that will do this.
You also might see CO2 manifolds around, where you can hook up multiple kegs to a single regulator, but as you noticed that means they are all at the same pressure. That's a fairly cheap way to set up multiple kegs, but that means you give up that flexibility.