Season five could POSSIBLY be the last season, but the producers are considering shooting two endings: one that wraps up the show if it's canceled, one that continues if it's renewed.
We'll see more of Nathan's past, and maybe what his greatest fear was when he looked into that Troubled girl's face in season two! Been wondering that for ages.
If season five is Haven's last (which is probably will be), James absolutely MUST return! They barely even mentioned him in season four, which is pretty ridiculous considering he's Audrey and Nathan's child!