Windows Live Wave 4 is finally leaked and available for download. WLW4 includes Messenger, Mail, Photo Gallery, Movie Maker, Sync, and Writer.
Messenger - Social integration, different views, expandable contacts
Live Mail - supports yahoo mail without premium service(need manual configuration), gmail(auto configuration) and other email services, bundled emails according to contacts, improved new UI, date and event sidebar.
Live Writer - Smilies added, improved new UI, supports plugin(including WLW v14 plugins), quick image editing.
Photo Gallery - quick retouching, tagging
Movie Maker - supports uploading video to Youtube, Facebook and Live sites.
I only mentioned several new feature here, feel free to explore the new feature and get it here
*unfortunately WLW4 doesn't supports Win7 Taskbar yet - tested on Live Writer and Live Mail*