Last November, Murphy first suggested that the fantasy drama could return for two more series.
He recently told CultBox: "We do have a general idea of what would happen in series five. We've always known a long while ahead where we wanted to go."
He also suggested that the show is unlikely to lose any regular cast members in the foreseeable future.
"I don't think we're likely to lose anyone in the cast that's key to our developing story," he said. "We know where we're headed and I don't think anything particularly will stand in the way."
Murphy also revealed that a number of possible Merlin spinoff projects have been discussed, including a cartoon.
"It's very early days, but there are thoughts and ideas," he said. "I think it'd be very interesting to explore. Something we've discussed, which would have to happen after the series had come to an end, is an animated series."
Merlin star Angel Coulby recently revealed to DS that her character Gwen will have a "changed" role in future episodes.
> Merlin Q&A: A chat with Katie McGrath