In October, that is the month that Activision is going to be pulling out all the stops trying to get people hyped for Ghosts. If they also advertised DLC for Black Ops 2 during that crucial time, it could detract from the hype around Ghosts. It's true that Ghosts has been advertised a little bit, but during October is will be heavily advertised and you want as little competition as possible when it comes to this kinda stuff.
By the way, yes a 5th DLC is impossible. David Vonderhaar confirmed on Twitter that they are committed to 4 DLCs, no more no less.