The zone boarding starts with windows, then middles, then aisles. If only middle seats were left, they would actually get zone 3 (not zone 4), and probably still have overhead space. Sadly, given the checked bag fees, there will be customers who have to gate-check on any legacy airline these days. On United, it's the aisle customers who are left with this, as zone 4.

As an aside, the "outside-in" boarding is considerably more efficient than "back-to-front," despite common conceptions to the contrary. See the following fascinating simulations:

http://leeds-faculty.colorado.edu/va...g/boarding.htm

http://leeds-faculty.colorado.edu/va...g.htm#section2