To quench my thirst for the main course, the second in the Lorien Legacies The Power of Six (which I have currently devoured - I must've been extremely hungry *dribble*), I thought it was best to get my hands on this novella. Although released well after I Am Number Four, Six's Legacy is supposed to be read as a prequel to the series. However, knowing about Six (the aim of this novella) or not knowing Six when you read I Am Number Four, is nothing to be concerned about and it doesn't matter either way. You basically learn most about Six through John (Four) in The Power of Six. Reading from Six's perspective though, gives you an enticing look into her past and survival first-hand. It makes a lot of sense if you read I Am Number Dour so that you are more familiar with the backstory that is referred to in the short. 
 The Power of Six is an explosive and well-rounded novella that gives you an insight into what it's like to be Six and where that feisty girl we know of, came from.