You can install both Xcode 3 and 4 at the same time: Xcode 4 is installed in /Xcode4 , and Xcode 3 is installed in regular /Developer .
However, keep in mind that if you upgrade the UNIX development kit, it will replace Xcode 3 versions and Mac OS X versions of some command-line tools with Xcode 4 versions.
For the most part, you will need to use Xcode 3 for any code header in the application store.
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