No more Xcode 4 for Snow Leopard?
The App Store only distributes one version of each app. Xcode 4.0 is no longer available as Xcode 4.1 replaced it.

If the author of the software wishes to keep an older version on the store, the new version has to be uploaded as a different sellable item.

In this case, Apple didn't choose to leave both.

You can get multiple versions of Xcode by joining the paid developer programs. Last time I checked, Xcode 3 was available with the free account. It runs on Snow Leopard - but it doesn't look like you can buy Xcode 4.0 anymore without a paid developer account - it might not even be available to the developers - you might ask before paying money if you really need 4.0 and only 4.0.