I think it's a game by game thing and situational. I wouldn't be surprised to see them return to that against certain opponents.
Yup, you're in the right time frame. They were defending the market with 5 underneath (rushing 3). That cover 4 could have worked, but would have left "the hole" a little more open for attacking. Still probably a preferable result though. I personally would have been happy with a traditional Cover 3, maybe run C3 buzzi if you're worried about the middle of the field being attack for first down.