A TED presentation by Siegfried Woldhek. Two of the four faces are already commonly held to be da Vinci (one as a child, one as an old man). Woldhek does some interesting comparisons to tease out other possible self-portraits, including the iconic Vitruvian Man.
A comment on the TED website points out a fifth likeness, a statue of Leonardo at the Uffizi in Florence, Italy.