I'd have to say there's nothing that will really spoil the fourth film, but to answer your question the only way would be to use a code because I'm pretty certain you unlock the mermaid around the fourth level into the fourth film.
"If you've got yourself a dream, work hard, believe in yourself because I know dreams can come true."- Dennis DeYoung 1997
actually I think it was the 5th level in the 4th movie, seems like the mermaid was the last freebie character I got. And yes playing the 4th movie will give you a very good outline of whats going to happen in the movie. If you have seen any trailers at all then those combined with this game will all but tell you the main story. So unless you mean spoiler as in tell you every detail & play by play it will indeed spoil it for you.
Yeah, I was worried about the 4th movie being spoiled for me, but then I asked myself what it would have been like if I had never seen any of the movies before playing this game. Look at movies 1-3, then look at the game counterparts- they're easy to follow along with if you already know what's going on, but, if you don't... they don't really give away a lot of info.
I found out only some of the most general details of movie 4, nothing too specific... except maybe the very end.
In order to make an apple pie from scratch, you must first create the universe. You musn't be afraid to dream a little bigger, darling.