At the end of season three, it was revealed that Queen Elsa of Frozen would be in the next season of Once Upon a Time. I heard the news despite never having seen the show. My mom mentioned she watched the first season and liked it so the news of Elsa made me want to watch the series. So I watched all the three previous seasons before the fourth came out. By this time, I had liked the series so much that it didn't matter if Elsa was in it or not.
Like the third season, the fourth is split into two story arcs. The first is about Frozen's Queen Elsa being released from a magical container and she enters Storybrooke scared and confused. It seems her sister Anna is missing, and so is Elsa's memories of how she ended up getting to Storybrooke to begin with. The mastermind turns out to be someone very similar to Elsa, and she has a grand plan both for Elsa and Emma Swan.
The second half is called Heroes and Villains. Regina and Mr. Gold realize that villains never find true happiness, so they both try to figure out a way to fix this mostly though their own independent ways. However the sea-goddess Ursula, the evil Cruella de Vil, and the dragon-witch Maleficent arrive in Storybrooke for the same thing Regina wants. Unlike Regina, they want to remain evil in their quest.
I really liked the fourth season a lot, and I didn't think I would like the Heroes and Villains part as much as I did. Though I did like the Frozen story better than the second half. Though I will say I didn't like the direction some of the characters were going, but there were plenty of good surprises and twists. I also wished they stopped doing so many flashbacks about Snow White and the Evil Queen. I have an optimistic outlook for the fifth season, but I don't think any fans will be unhappy with the fourth.