I have two iOS projects that under Xcode 3 built properly and these iPhone been on sale in the app store for some time. After upgrading to Xcode 4, Xcode now thinks these are Mac OS projects!
When I select Project Settings > Info, it actually says at the top "Mac OS Deployment Target" with a drop down allowing me to select 10.4, 10.5 or 10.6! These are not nor ever have been Mac OS projects.
In my build settings, Xcode allows me to select 32 bit or 64 bit Intel architecture. not armv6 or armv7.
You need to change your build settings. Fortunately, this is really easy to do.
Start by clicking on your project's name in the Project Navigator.
Now, click on your Project Name as it comes up in the list. (You may need to do this for your build as well.)
Next, click on Build Settings. Finally, you should be able to change your Base SDK to the appropriate one.
Note that you might need to repeat steps 3 and 4 after selecting your Target in the screen shown below.
Have you restarted Xcode since the upgrade? Often times Xcode 4 seems to need to be restarted after SDK changes.
I didn't face any issues opening the projects in Xcode 4 which are built in Xcode 3. However there can be wired issue with the Xcode 4 you are using. Try resetting the architecture to armv6 or armv7 and base SDK to latest iOS SDK using the build settings. Also set the deployment target properly. This should allow you to run the application in Xcode 4.
Also, try uninstalling the xcode and install Xcode 4 again. This should fix the wired issue you have with your xcode 4.
来源：https://stackoverflow.com/questions/6586675/xcode-4-my-iphone-projects-have-become-mac-os-projects-and-i-cant-change-thi