Without disclosing how they came to that number it's just marketing puffery. I doubt it would hold up to scrutiny. But even 4 million active Ubuntu desktop users would be impressive.

The blog entry at:

http://jspaleta.livejournal.com/42464.html

estimated (in 2009) the number of Fedora users at around 16 million. I think Ubuntu has more users than Fedora had then.