I would like to add that the Season 3 finale actually matched Season 2’s finale ratings, which is actually not bad. But yes, there will be more riding on the ratings this Season. Especially with all of the Frozen hype, which I’m sure ABC is hoping will draw in more viewers. I think if the ratings are at least not lower than Season 3’s, and if Once still beats its direct competition, a 5th Season is likely to happen. Whether a 5th Season is the last depends on both ratings for this Season and various factors. For example, some of the key actors might want to leave the show to pursue other projects, given that their contracts could be expiring around their 5th year. Pretty sure at least Robert Carlyle’s contract is 5 years.