The fourth city relics are now dropping from level 7 – level 10 goblin camps.
Here is how you can farm them in no time.
Be sure to have around 400-600k Tier 3 troops. 100-200k of each kind, mounted, foot and ranged.
Have at least 2-3 high level heroes.
Start by searching for a level 7+ goblin camp. Once you have found one, bookmark it and send your hero+troops to attack it.
After you do that, search for another goblin camp, do the same thing, bookmark and send your second hero+troops to attack.
Do the same thing for a third goblin camp, with your third hero.
This way you won’t waste too much time waiting for your hero to come back (assuming that you want to use only one hero).
If you don’t have too many troops, either build or farm the relics in the old fashion, with one hero. It will be slower though.