The creator of Being Human, Toby Whithouse, has spoken to EW about Series 4 and the death of Mitchell. Here’s some highlights

Did you have to kill Mitchell?
As it was, we had Aidan for series 3. He got offered the [Hobbit] toward the end of filming that [season], so I hadn’t written the last episode yet. I think even the shooting script had a number of different endings, including one where he gets whisked back to Bolivia with the other vampires. As a matter of fact, it was really Aidan who pushed for Mitchell to be killed, because he thought that was the appropriate ending for the character.

But when EW interviewed Aidan in February, he said he’d be willing to return to the show if the dates could be worked out. He must have known by then Mitchell was doomed.
We mentioned to him that there might be a chance of bringing him back in flashbacks. We’ve done them before. But ultimately those things sound very well, but in practice they are very complicated. We’d have to secure him so far in advance, and with the way The Hobbit’s filming dates keep shifting around, we’d never be able to guarantee that we’d properly get him. Somebody like Aidan, God knows we’d absolutely love to have him back, but because our budget is so small, we have to be pragmatic. We have to acknowledge that our chances of getting him back are pretty slim.

Are you going to add to the cast in season 4?
Lee Ingleby [as 1,000-year-old vampire Wyndam] is coming back for some of Series 4. We’re also bringing back Tom [Michael Socha], the young werewolf. The cast of the show is constantly changing and evolving and growing and shrinking. We’ve been very lucky that pretty much every single guest actor we’ve had, as they’ve been leaving, has said “You know, I could come back if you wanted me to.” Being Human is a bit like a Mexican daytime soap — even if someone dies, it doesn’t preclude them from coming back at some point.

Whithouse also revealed there’s a chance that Ivan (Paul Rhys) and Adam (Craig Roberts) could also return, “If a story comes up”. Full interview here.