It was actually 4 and a half. Innistrad came out in 2011. RTR, Theros, Khans, and Battle being the blocks in-between. The Zendikar return was after 6 years so its not off by THAT much.
Between the release of Avacyn Restored, aka when we were leaving the plane, and this announcement, it's been 3 and a half years. In comparison to the time between RoE and BFZ, Dissension and RTR, and Fifth Dawn and Scars, this is the fastest we've gone back to a plane. It will be 4 years when Shadows releases.
There's nothing wrong with that per se. It's interesting that we're putting Chandra's plane on the backburner, though.