I've just noticed this ebook came out a couple of days ago. It's 144 pages and is available on Kindle and as an epub (UK: £1.49):
Pittacus Lore's I Am Number Four: The Lost Files: Six's Legacy
Number Six - when John meets her in I Am Number Four she's strong, powerful, and ready to fight. But who is she? Where has she been living? How has she been training? When did she develop her legacies? And how does she know so much about the Mogadorians? In I Am Number Four: The Lost Files: Six's Legacy, discover the story behind Six. Before Paradise, Ohio, before John Smith, Six was traveling through West Texas with her Cêpan, Katarina. What happened there would change Six forever . . .