After long time trying, I found that the unique way to download Xcode 4 which runs on Snow Leopard (Mac os X 10.6) is becoming member of iOS developer program.
Actually, Xcode 4 could be downloaded both through the web page of apple and through the app store. The problem is they only has the last version, 4.2.1, which only works for the last version of Mac OS X Lion (10.7). If the user doesn't have updated to this one, what she can choose is use the version 3 of Xcode, which doesn't match with many tutorials.
Thus, if you want to learn from the new tutorials and you doesn't want to update your OS, the unique way to obtain Xcode 4 for Snow Leopard is paying 99$ and then, have access to the resources of develop center.