Older threads mention that there is a download for Xcode for Snow Leopard listed in the Xcode downloads on https://developer.apple.com/downloads/index.action?name=Xcode if you are logged-in as an enrolled iOS developer - but I can't see it.
A comment on this thread has a direct link to the download, but clicking on the link redirects to an "Access Denied" error page:
https://stackoverflow.com/questions/7662246/cant-download-xcode-4-for-snow-leopard-anymore
you can refer to here. or if you have a good internet connection, try to download via torrent. here is the link. link for xcode via torrent.
Your Required software is available at this link, https://developer.apple.com/downloads/
Currently Xcode 4.2 is on 6th page.
Please note: OS X 10.6.8 (snow leopard) is now TOO OLD to publish an app.
You need a mac with OS X 10.8 (mountain lion). Apple have not maintained backward compatibility: the minimum versions of Xcode or Application Launcher needed to meet the app store requirements won't run on 10.6.8.
You can develop, test and simulate with your old mac, but ultimately to build and publish your app you'll need a newer Mac.
来源：https://stackoverflow.com/questions/12243710/xcode-download-for-snow-leopard