If you're part of the OS X 10.10 Yosemite beta program, you can now grab Developer Preview 4 from the Mac Dev Center. What's in the update is hidden behind the veil of the nondisclosure agreement, but members of the developer preview program can now download and install the latest version of the beta to both test it for bugs and (perhaps more importantly) develop new and updated apps that take advantage of the new stuff that Yosemite has to offer.
If you're in the developer preview program, the beta should now be available to download through the Mac App Store.
Though Yosemite is currently in a closed beta, Apple is expected to release a public beta for the OS later in the year, once we get closer to launch (which is likely only a few months away).
In the meantime, what new features in Yosemite are you most looking forward to?