Zones 4+ are not from back to front, but rather delta scatters people throughout the plane. Zone 4 will most likely be towards the back, but the rest of the zones are scattered. There was a study done and they found this randomness actually fills the planes faster as people can be boarding through out various areas of the plane at the same time, thus reducing the isle blocking and reducing the amount of time it takes to board. At least that's how a FA told me it works.