Take a look at the Android SDK manager. You will have some default modules which were installed with your SDK. You can download and install other modules which correspond to all the incremental releases of Android SDK.
When you create your project, you can specify a particular Android version to target.
It helps to understand that there is no "Android SDK 4.0.x". There are versions of the SDK that support Android 4.0.x, like version 15 and 16. So far, all SDK versions that have come out have continued to support Android versions that came out before. I think the only Android versions no longer supported by the SDK are 1.0 and 1.1.
As long as you set the <uses-sdk> element in the AndroidManifest.xml file correctly, and test your app in a 2.3 emulator environment, you should be fine.