According to BBC Sherlock creator Mark Gatiss is Benedict Cumberbatch irreplaceable as Sherlock. Martin Freeman is in my opinion also irreplaceable as Dr. Watson.
Due to the busy film schedule of both Benedict and Martin they are able to do a series of three episodes not more then that.
The rumours Mark Gatiss started right after the last episode of the 3th series in the UK that the new 4th series was planed for Christmas this year. It came as quite a surprise for fans. We have been waiting a long time for season 3. But unfortunately he could tell us a couple of weeks later that it wasn’t an option due to busy schedules of both the stars of the show. Now new dates had been placed for 2016.
Maybe the wish from Mark Gatiss will come true in 2016 he always have wanted a Sherlock on Christmas.