AsNoTracking() Method Is Missing From Context in Entity Framework

I need to load an entire table into memory using Entity Framework 4.0. I have spent the last 2 hours reading about the AsNoTracking() method that should do the trick but, I cannot figure out why the method is not available on my dataContext. Based on everything I have read, I should merely need a reference to System.Data.Entity. Then, I should be able to use the AsNoTracking() method when loading my objects. Am I missing something simple here? Is this method not available in EF 4.0? Nevertheless, below is one of the queries from my code.

// Working Query
var items = dbContext.Items.ToList()

// Does NOT Work (Compiler does not recognize AsNoTrackingMethod() )
var items = dbContext.Items.AsNoTracking().ToList()