4.0.2 is still current on the developer download page and will work with Snow Leopard. If you want to use the Lion specific features (full screen mode) then yes you need to update to Lion.
Thank you for the comment. I remember seeing the Xcode 4 being available only for paid developer accounts mentioned somewhere else, too. After I given up hope downloading it, I've searched my Firewire HDD (or I should better say a Seageate FreeAgent Pro with USB & Firewire) and luckily there I've found the xcode_4.2_and_ios_5_sdk_for_snow_leopard.dmg which I had downloaded in 23.December.2011, I've installed it by setting the system time to 1.1.2012 and it works quite fine.
Thank you for the comment. I knew that site and I actually used it to locate the Xcode DMG file in question but for that particular version of Xcode it's been no help, the links simply didn't work. But take a look at the comment above, to my luck, I've found that particular Xcode version in one of my HDD's.