The damage is reduced to 6, the max for a minor.  If the damage was 4, it would have stayed at 4.
Side note: I still think there is a little flaw in the rules there.  A heavily industrialized territory bombed to max (20) dmg, needs to spend 11 ipcs to build one unit and a lightly industrialized territory bombed to max (6) dmg can do so with 4 ipcs.  It seems odd that a less industrialized territory can recover more easily. Maybe majors should be limited to 13 damage?  Or for every 2 points dmg production is reduced by one.  So 1-2 damage = 9 capacity, 3-4 dmg = 8 capacity, etc