I just download SDK4, but when I compile a project, I found there is "Base SDK missing" on Simulator selection, though there are iPad 3.2 Simulator and iPhone 4 Simulator.
Usually you just want to set the Base SDK on the General Tab of the Project Settings pane. Then all the other places it can be set should just inherit that value. If you set the Base SDK on the target settings pane then you have more places to change when you need to change it again.