IKYWT was a monster smash. However at this time of the year in 2013, it had sold about 3 million in the US. So BS and SIO are already outselling it (in the US) at the same stage of the current era.
Next week Taylor will become only the second artist in Soundscan era (since 1991) to have their first 5 albums sell more than 4 million copies in the US. Garth Brooks is the only other artist to achieve this record!
ICannot was talking about how 1989 is reaching this milestone much quicker than Taylor's other albums in order to get some perspective of it's outstandingly quick sales. I would agree, and add some extra context by saying that 4 million copies in the US is a milestone that most acts can never reach even once.
I would argue that only Taylor and Adele can even reach it these days. Disney can have a smash once every 20 years, like Frozen Soundtrack and maybe Michael Buble with his smash Christmas album but who else? Eminem and Lady Antebellum have done it once this decade, but can they ever do this again. I doubt it, considering the sales of their latest albums.
The mark of a really successful album in the US these days is 2 Million. So Beyonce, 20/20, Unorthodox Jukebox, TTAL, MMLP2 are regarded as being smashes because they sold more than 2 million these days. Even in her lowest selling albums, Taylor can reach double that milestone! She is so far ahead of the others it is incredible.