I really wish I had seen this before the last post. It seems Russell T Davies and Julie Gardner had a lot more to say, though excessive typing isnt a job for a Sunaday afternoon, so I am going to be lazy and break it down.
Firstly, Julie Gardner has confirmed that filming on Series 4 will begin in Wales. "The Welsh setting still features in a really significant way as we move forwards so when we start filming in January, we will start filming in Wales. There will be beautiful landscapes. Gwen Cooper is still that character. She's still Welsh. Her story will take place in the U.S. but also very much in the UK, so that flavor of Wales will be very apparent."
Russell T Davies then confirmed that Rhys will be making a return to the show, with his and Gwens child in tow, and also hinted at a possible fifth series of the show. When asked whether Captain John would be in series 4, as had been speculated on, RTD announced that although James Marsters won't be in the upcoming series, he is still in contact with the actor about a possible future appearance, which would indicate that he at least has a a direction in mind should a fifth series be picked up.
You can read even more (without the lazy breakdown) over at Zap2It