Firesinger (The Elemental Destinies, #4)

Meet the Elementals - a magical race that inhabits our world, embodying the four elements of earth, air, fire, and water.
Hope is what they call a "Perfect Impure," a forbidden combination of all four elements never seen before. Afi is the Firesinger sent to hunt her down. They must both battle the forces that surround them--forces that threaten to tear the world apart and reduce it to ashes.
Together they'll learn that love is a burning thing, and the heat between them cannot be denied.