The third and fourth levels of each world each contain a boss fight at the end. (thatvideogameblog.com)
Rise to new heights of power with an increased level cap, taking your hero to 70 and unlocking new skills (as well as a fourth (shopto.net)
«After monitoring the volcanic activity of Mount Agung in the past month, the PVMBG recommend downgrading Mount Agung's alert status from beware [fourth level] to alert [third level]. (thejakartapost.com)