By the time you've hit Rank 4, you as a Sovereign have probably got a solid social arrangement holding up your Rank. You've already got a decent processing infrastructure, or you're working on one as you skim this post. Chances are you've got some Slabs and have been preparing to start construction on your Inner Courtyard. This will unlock the rest of your Rank 4 district options.
Like most default districts, the Inner Courtyard adds another seat to the Department of the Interior. The Great Hall is one of the first great impressive buildings. As it Ranks up it grants the settlement a higher and higher Respect bonus, increasing how cool your whole population thinks you are. It also provides beautiful rooms to a few Players per level and processes grain into beer.The Orchards, which can be built no-where else, provide Olives and Produce, store a whole lot of carts, and increase settlement-wide storage. This is not a bad deal for a required district. The District Action is Dine in the Hall which increases local Toughness and Peasant population.
This is a great district if you like Boats, Ships, and Prosperity. These concepts often go hand in hand for wherever there's water trade, there's an opportunity to make some cash. The Harbor build and store Boats as well as processing Lumber in to Boards. Again, useful for boats. The exclusive Floating Docks provide storage space for both Boats and Ships and increases training speed and luck for boats. The Lighthouse can technically be built anywhere and is a great local Prosperity booster, on top of increasing local ship luck. Building this district also unlocks Naval Recon which allows players to peek in on the economies of neighbor settlements.
The Port District is a transportation-central district. The main Port Authority Office isn't water-craft exclusive. It can craft and recruit crew for Boats, Ships and Carts as well. The Exclusive building, the Cargo Terminal, can store the three types of transportation unit it's main building can produce. It also provides a percentage bonus to the income brought back by these units for every Cart, Boat and Ship in the settlement. Starting your Port district requires nothing but a large stack of paper in preparation for all the accounting that will occur. This district provides the Work as Stevedore action which allows characters to trade action points for currency and possibly goods.
The Field of Mars is an incredibly functional member of the Department of Defense. This district mainly focuses on preparing a settlement to produce many quality troops. The Main building, the Piazza Navona, is another classic sink for Slabs but offers several useful functions. This building enhances housing space for all NPCs throughout the settlement and processes sandstone into bricks. The Exclusive Praetorian Assembly increase Commander tax efficiency and are always protected by 2 free Legionaries troops per building level. Training Grounds, which train all types of troops but Monsters, are most efficient in the Field of Mars. Players in this district can perform Martial Training, temporarily increasing the attributes of all local military units.
The Central Commons is all about Bread. It's also about Beer and maintaining a high and happy Peasant population. If you need to keep your Peasants happy and fed, the Central Commons is a good investment. The District Action primarily incrases the local Peasant population based on the acting player's Charm attribute. The Main building is the Dining Hall which processes grain into Bread, provides player rooms and increases the rate Peasant s move into the settlement. The Exclusive Tavern can house a smaller number of players per level and the linked Bakery just bakes that bread.