DC Comics announced yesterday on The Source blog that Flash: Rebirth by Geoff Johns and Ethan Van Sciver will run six issues now, versus five. They also shared some info on the fifth issue:

In issue #5, which will be solicited in the June Previews, the greatest threat to face the Flash Family in decades stands revealed. Plus, a new hero will take on an older speedster’s role. And Barry Allen faces having to once again sacrifice his life to save all that he holds dear.

They don't mention a reason why the mini-series was extended, but maybe someone will ask about it in the next Geoff Johns Prime on CBR ...