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.