According to Autoblog all this speculation may be much ado about nothing. They say they have reports that it is simply an LP640 Superveloce
In ways, it makes sense... an all-new model from Lamborghini seems like a Geneva, maybe Detroit debut. It would really surprise me if they chose Paris to debut something as big as their first 4-door.
Although comparing the teaser to a rear shot of the LP640
shows pretty much zero resemblance, so that would be a pretty heavy styling change for an SV model.