How to restrict the flex iPhone app only to iPhone 4 and above

Look for the UIDeviceFamily key in the inforadditions part of the app-descriptor xml. That's an array, and currently you have it set to include 1 and 2. Remove the 2 part, and that will cut out iPad.

You should realize that many things work better, or at least as good, on the 3gs as they do on the iPhone 4, because the Retina screen is dealing with 4 times the number of pixels. If you have some other reason to cut out the 3gs, you could require that the devices has a front camera. See this article for how to do that:

http://help.adobe.com/en_US/air/build/WSfffb011ac560372f7e64a7f12cd2dd1867-8000.html

One thing though, if you don't have any feature in the app that requires a front camera, Apple may decide to reject the app. You should at least test the app on 3gs and the later iPods, it may well work fine. Also, you could add something to your notes in the iTunes page to strongly recommend iPhone 4 or 4S.