Apple has released OS X 10.10 Yosemite Developer Preview 4 Update 1.0 to developers for testing.
The update can be installed via Software Update on your Mac. Apple says “This update is recommended for all users.” It’s should also be available on the Mac Dev Center shortly.
Apple also released Yosemite Recovery Update 1.0 today that includes improvements to OS X Recovery and is recommended for all users running OS X Yosemite.
If you aren’t a developer and you want to test the new operating system you can register here.