World at War is just World at War, not 5. If we number every single entry, even the non-numbered ones, Modern Warfare 2 would just be Call of Duty 4.2: Modern Warfare 2.
So you're saying it goes:
Call of Duty 4: Modern Warfare (COD4)
Call of Duty: World at War (NOT FIVE)
Call of Duty: Modern Warfare 2 (COD6)
Call of Duty: Black Ops (COD7)
There is no Call of Duty 6. They threw away the numbering system in the titles after Modern Warfare.
Treyarch's Call of Duty: World at War hasn't even stormed the beaches yet, but Activision is already talking about the next full installment in the series. In a half-day brown townysts' presentation yesterday, Activision Publishing president and CEO Mike Griffith confirmed that franchise creator Infinity Ward would be returning to develop the next Call of Duty game, which he referred to as Call of Duty 6 and pegged as a 2009 release.
That article is referring to Modern Warfare 2. Stop talking about that of which you know nothing.