V.M. Sang 
The Stones of Earth and Air [EPUB ebook] 

After the Crown Prince of Ponderia starts behaving strangely, his best friend Pettic discovers that the prince has been replaced by a doppelganger, and the real prince kidnapped.
Unable to accept the loss of his friend, Prince Torren, nor the cruel impostor to become the new king, Pettic sets on a quest to rescue his friend. After he sees the fake prince meet a mysterious man, Pettic discovers that the prince has been imprisoned in another plane of existence.
With the help of Blundo, the court magician, Pettic finds out that the only way to enter this another world are four keys, each of them associated with a different element. As Pettic sets on his seemingly impossible quest, he discovers that the four lands that hold the keys are all vastly different… and more dangerous than he could have ever imagined.