She is a Vipperloid made によって vip@ch. She is created as a joke to trick the Nico Nico users into believing that she is the new Vocaloid. She is released on April Fools. The number, 04 is later changed into 0401, it means April 1st. At first she doesn't have a voice. This video explains it all.
So. Teto is a vipperloid/utauloid, but people are talking about making her an official vocaloid. But anyway, what does it matter? Utau and Vocaloid are the EXACT SAME THING, with different vocalists. O really, does it matter?
No, Teto was originally never a vocaloid, but only a troll vocaloid (as some may call a boucaloid) She was 100% fanmade, but people thought crypton was actually going to release her as a new vocaloid, but instead crypton used her voice bank for their uses only, later on they released Teto'd VB as the "Tougou full,CV/VCV" and the "Tandoku lite, CV only" (This might not be accurate) o3o
Well there is a logical answer for this, Since Teto is first created on 1/4/2008 she isn't the fourth character vocal. Megurine Luka (known as CV-03) was first produced on 1/30/2009 so we still don't know who is the fourth character vocal there is.