User blog comment:Jimeee/Skyrim 1.9 beta update adds Legendary difficulty and raises level cap (Updated Apr 8th)/@comment-97.106.15.105-20130322231203/@comment-5469467-20130323025418

Skyrim is technically the fifth game in the series. It's there in the title: The Elder Scrolls V: Skyrim, where "V" is the Roman numeral for 5.

That said, I'm thinking the original post may just be refering to the most recent three games, and was comparing Skyrim to Oblivion and Morrowind.