Well 1 is the default Admin user, and 3 is Satoshi, so i bet 2 might have been another user he/she made and then later deleted, unless SMF has a second default user (moderator?) in any case its long gone...
Okay, if this is the case so the question should be changed to "Who is the 4th bitcointalk user?"
It would be interesting to know who joined the forum after Satoshi. Maybe one of his friends? This might also solve his identity issue!