I've recently discovered a site that embraces many of the suggestions put forth by this community of what Daily Kos 4.0 should be. It's called Change.org, it's a social network for social change, and it's truly inspiring.
Change.org supports basic social networking features such as user profiles, friends, personal blogs, photos and videos. But it's centered around promoting social causes, such as stopping global warming and promoting universal health care. These causes appear in a tag cloud on the home page.
Each social cause has its own community blog where users discuss the issues of the day, as well as a Digg-like news system where users submit stories and sites from around the web and vote for the ones they think deserve promotion. In addition, there are tools for creating projects and taking action, plus fundraising systems to support such projects. Local networking is supported by geographic tools and maps, facilitating local action. These tools would be just as effective for DK4's presumably heavier emphasis on electoral politics.
Change.org doesn't yet have a formidable first-mover advantage over a potential DK4. I believe DK4 can leverage the size and intensity of the Daily Kos community to create the preeminent site for political and social action. But Change.org provides inspiration. Take the video tour.