My 4 year daughter has dark circles under her eyes and is always saying she is tired she sleeps about 12 hours a night what could be the reason?

Many reasons: The dark circles could be an inherited trait, the result of allergies, sleep apnea, anemia, sinusitis, lymphedema, and many mother things. It's inappropriate to, ascribe this just to, a "junk" diet. See your pediatrician for a proper examination and explanation (and treatment if needed). Most common reason is allergy, by the way.