http://www.youtube.com/watch?v=rlygadz4Lmo
Trailer for series 4
As we already knew, Catherine Tate will be around for about half of series and Martha will possibly be around for the last 3 episodes or so.
The Ood are returning but probably not to any great effect, due to them being a slave race.
At least 1 episode seems to be set in Roman times.