@Steamedtoast
Actually she can't tell them, as in the novel the king (her uncle) put a magic like curse upon her, usually this magic is meant for assassins or spies (something to that affect). But anyways this magic that's casted upon her doesn't allow her to speak of the secret she holds (her being a girl and an imposter sent in by the king, in place of his sick son; the crown prince) or she would die immediately upon revealing this secret, by bleeding violently through the mouth.
I understand, why it wasn't explained more elaborately in the promo, but if you read the first few chapters of the novel (which I did with the help of google translate, which is a bit rough on translation) you"ll understand why she cant explain to the 4 dudes or anyone of that matter.