Zones Unlock when you complete prequisite zones. to unlock the fourth zone named Blackhill that looks like a granary on the map, you must complete Darkwood Forest (1 total zone) Looks like a tree on the map, Chestnut Grove (1 total zone) Looks like a broken building on the map. Then you must complete Bone Meadow (5 total zones) Looks like a mound of dirt with a sword and a flag in it on the map.
The first thing I would do in your situation is select BoneMeadow from there you will see a description of the zone "A great war here a century past claimed many lives." below that you see a red bar telling you what zone you are selecting. if it says 'Zone 5' That means you either just completed up to 'Zone 4' or you completed 'Zone 5' already which means you have finished BoneMeadow. If it says 'Zone 1' or 2-4 I would finish those zones including 'Zone 5' to complete BoneMeadow.
If it says 'Zone 5' and you are like, duh man I already did it!! Then I would select Darkwood and Chestnut Grove, and finish both of those zones, they are short and can be completed with 30 energy.
This should unlock the fourth area Blackhill 'Zone 1'
Hope this helps! feel free to submit the invite ID X6ndP (case sensitive) Thank you!