Shacknews has three PC screenshots and two Xbox 360 screenshots of Quake 4. You can watch them here
And IGN had an interview with id Software CEO Todd Hollenshead about Quake 4, read it here
It's a continuation of the Quake II storyline, and it picks up right as Quake II ended. If you remember, Quake II ended as the last thing you see is a guy stick his fist out of the escape pod after killing the Macron.