One of Time magazine’s top 50 Web sites of 2011 and the brainchild of Diary of a Wimpy Kid creator Jeff Kinney, Poptropica is celebrating its fourth birthday this month. On Friday, Poptropica, which is published by Family Education Network, a part of Pearson, will launch its 21st island, Mystery Train, which enables kids to travel back in time to the 1890s and the Chicago World’s Fair, where they can meet historical figures such as Mark Twain, Thomas Edison, and Susan B. Anthony.

“When we launched Poptropica, our goal was to share stories with a generation of kids empowered with digital and gaming literacy,” says Poptropica publisher Jess M. Brallier. “In four short years our simple notion reconfirmed what every generation of kids truly values. The concept has exploded into a worldwide business phenomenon.” Up to now that’s primarily meant toys, available at Toys “R” Us, and game cards. But in early 2012 Penguin will launch a Poptropica imprint with novelizations of the individual islands.