Try searching Netflix on the sub and read one of the many posts :)
Here's a few that might help...
https://www.reddit.com/r/TheLastKingdom/comments/grfdij/i_cannot_watch_season_4_on_netflix/
https://www.reddit.com/r/TheLastKingdom/comments/gptywm/cant_find_s4_on_netflix/
https://www.reddit.com/r/TheLastKingdom/comments/g9j8bi/why_am_i_not_seeing_s4_on_my_netflix/