Indeed, they will use "series" rather than "season" over 4.
When I hear about the diff. BBC networks I'm reminded of a gag in Monty Python where a bunch
of funeral home workers want to listen to something on the radio. "I think it's on Radio 4".
Opens up a big radio, takes out a smaller one "Radio One, radio two..."
Inside the smaller one is an even smaller radio. "...Radio 3..."
And finally the smallest radio, is inside of that. "Ah! Radio 4!"