User blog comment:Aizen.sama/Are Espadas, Vasto Lorde(s)?/@comment-1769234-20100422003851

Well that is true no one other than Kudo knows though this how each one of the top four were killed Starrk was killed by Captain Kyoraku a captain with powers close in strength to Yamamato. Barragan was killed by his own aging power. Tia was killed by Aizen and Aizen's spiritual power is at least that of two captains. Uliquorra died to wounds by a full Hollow Bankai Ichigo.

Though the Silhouette of a Vasto Lorde that Toshiro shows in the anime looks like would have been Uliquorra before he became an arrancar. Though my belief is that the top four were Vasto Lordes but only average or the weakness ones there could be much stronger ones. And Aizen states that he is stronger than any of the Espada, point given when he kills Tia the Tres Espada.