The 4th City in Glory of Rome requires a player to be level 10, control a plain and be in possession of a 'Fourth City Deed.' A 'Fo urth City Deed' can be obtained either through the shop, or by completing the "Earn Your Fourth City Deed" quest:
This is the text from the quest:
Find 20 Augustus' Marks, 15 Tiberius' Marks, 9 Caligula's Marks, 4 Claudius' Marks, and 2 Vespasian's Marks from attacking Wilds".
You can find the Tiberius, Caligula and Claudius Mark by attacking level 2-10 wilds, I would personally recommend hiting level 5 wilds. Please comment below if you have any experience encountering Vespasian's Marks. Reportedly, wilds of higher levels yield more/better marks.