So what's a Biology then?

Recently, a DVD box set containing all four movies of the Alien series (plus extras) was released. Nice, but there's one little detail about it that I couldn't help noticing: the title, 'Alien Quadrilogy'.

Quadrilogy?

Googling for this word while excluding the word 'alien' yields only a few hundred results. Not surprising, since according to the dictionary, it doesn't exist. The correct term for a four-part series is tetralogy, 'tetra' being the Greek for 'four'.
It will be interesting to see if the popularity of the Alien movies will manage to introduce this word into the English language.