The fourth Hokage.

What the hell do you think I mean? [img]i/expressions/face-icon-small-tongue.gif[/img] Itachi said that after Sasuke is able to use mangekyou, he would be the 3rd person to be able to use it. Seeing as how Itachi had no idea that Kakashi would some day develope his own mangekyou, he was not referring to Kakashi, but another person. Kakashi would be the 4th user of mangekyou.

I am kind of curious to know how Itachi knows all this information.