Cubase 6 upgrade from 4?

I cant see an upgrade from Cubase 4 to 6 listed anywhere in the shops, I am sure they all showed an upgrade from either 4 and 5 to 6 the other day but now its just 5 to 6 or Studio 4/5 to 6.

Which upgrade would I buy to go from 4 to 6?

Scrap that… its labelled an update not upgrade from 4 or 5… hence my searches showing anything :confused: