Since you have to use all of the 4 characters, it shows that the top right is a blue heart, and the spot below it is a plain blue square, meaning it has to be a blue character. The two uncovered characters are the heart and the star, meaning this lower right corner has to be a pink star if you know what I'm saying. Since two of the squares are blue, the others have to be pink and the shapes are already signified, making the top left a pink star and the bottom left a pink heart. Either I just gave you the answer and you can decode the process with that, or you won't understand this at all because I'm horrible at explaining stuff. Hope this helped :3