Zune 4.0 now support Zune HD, new online services, several key new features designed to make discovering new music and playing music in your collection even better.

-Quickplay is a new screen in the Zune software that gives you quick access to your content.
-A Smart DJ “mix” automatically creates a playlist of music from your collection - but goes the extra mile by ALSO mixing in music from Zune Marketplace.
-Mini Player Mode allows you to keep the player always on top of other applications while browsing the Web, checking email, or whatever else you might be doing on your PC. 
-Zune software takes advantage of key Windows 7 features! Using Jump Lists in Windows 7, you can access albums you’ve “pinned” under Quickplay as well as your preconfigured Smart DJ mixes of 5 of your favorite artists or bands. It also lets you shuffle all music and resume playing music if on pause. With Taskbar Previews (just hover your mouse over the Zune button on the Windows Taskbar in Windows 7), you can move to the next song, go back to the previous song, and pause your music and rate songs! And with Aero Snap, you can resize the Zune software to the top or bottom, left or right of your screen.
-Zune also features for the first time download-to-own, or rent full movies in combination of their growing TV show library. These movies and TV shows can be downloaded in standard definition (SD) or high definition (HD). 
-Zune Pass is only $14.99 and now comes with unlimited access to streaming music online! Oh and you also get 10 MP3 song credits a month to redeem too.

More info: Press Release | Download: Zune 4.0