Maybe you're thinking specifically about Enterprise drives.

Because Seagate were the first to announce and release a 4TB hard drive, almost 1 year ago (end of October 2011). They just don't currently market the drive as retail integrator (bare) product and sell it only in their own enclosure.

Hitachi was the second to announce, back in December 2011 also with encased drives and one model of bare drive, supposedly shipped in January 2012, but I think it might have only hit the street in March 2012. In April 2012 they announced the first Enterprise model.