As with previous SimCity titles, SimCity 4 places players in the role of a mayor (or several mayors), tasked with populating and developing tracts of lands into cities, while fulfilling the needs of fellow Sims that live in the cities. Cities are now located in regions that are divided into segments, each of which can be developed. The player has the option of starting the city in a segment of any of three area sizes, in real measurement the smallest has a length of 1 kilometers on a side, and the largest has a length of 4 kilometers on a side. A large city is 16 km²; for comparison the New York borough of Manhattan measures about 90 km². The size of a region and its layout of segments can be changed in a bitmap file provided for each region.
Upon selecting a specific segment in a region, gameplay is divided into three "modes".
The first is the God Mode, which allows players to design or terraform a selected tract of land where the city will be built. God Mode also allows players to trigger disasters, including tornados and earthquakes among several others. Players can select an area where a disaster will occur and even control the direction of certain disasters. Most terraforming tools are disabled after the city is named and founded. However, the player still has some terraforming tools (although they become very limited and expensive) and can still trigger disasters at will.
The second of the modes is the Mayor Mode, the fundamental mode of the game where the actual city building is conducted. Areas of land can be zoned as residential, commercial or industrial areas where the city will begin to grow. Players can build transportation networks, which include roads, streets, highways, subway lines, and bus stations. Other things that players can do in Mayor Mode are build civic buildings such as schools, hospitals, parks, police stations, fire stations, and public utilities.
The final mode is the My Sim mode which enables players to create user-defined Sims, which will live and work in the city the player has created. This mode can be used to closely assess citizens' needs through sims giving the player feedback. Players can choose a selection of characters or import them from The Sims.
Civic and utility structures
The functions of civic buildings have been overhauled in SimCity 4. Facilities that had previously provided citywide coverage, such as educational facilities and medical facilities, have now been modified to provide a more limited coverage, as it has been with police stations and fire stations in previous SimCity titles. Players can plan the best locations to provide sufficient civic services to Sims most effectively, for example placing schools in or around residential areas. Funding can now be adjusted for individual buildings, allowing users to specify how much money should be spent to supply a service in accordance to the local population. There are two sizes of police stations, fire stations and hospitals.
Maintenance expenses for public utility facilities (power plants, water plants, and garbage disposal services) will increase as they age. The maximum output of facilities also decreases as they get older. The rate at which facilities age is dependent on the percentage of its capacity being used and the level of funding being given to it.
Zoning and building occupancy
Zoning and building size have been improved for SimCity 4. Agriculture is now a separate industrial zone-type, enabling for farms to grow regardless of high land-value, so long as there exists demand for agriculture and agricultural zones have been provided. Zones are now automatically aligned towards roads; streets are automatically created when zoning on large tracts of land. Buildings are now classified into several wealth levels, zone types, and building size stages, which are affected by the region's population and the city's condition. The game simulates urban decay and gentrification with buildings darkening accordingly. Buildings originally constructed for occupation by higher wealth tenants can now support lower wealth tenants in the event surrounding factors forces the current tenants to vacate the building; this allows certain buildings to remain in use despite lacking its initial occupants. Buildings and lots can now be constructed on slopes.
Buildings in SimCity 4 borrow heavily from early 20th century architectural styles, particularly Art Deco and Romanesque Revival, while houses can appear in a traditional American Craftsman style. However, there is an additional, more modern, architectural style similar to Houston's architectural style. There are a number of buildings based on those found in San Francisco, including the Shell Building (appearing as "Wren Insurance"), 450 Sutter (appearing as "Vu Financial"), and the Pacific Telephone & Telegraph Building (as "The Galvin Corp"). Three of the game's bridges are also based on the Golden Gate Bridge, the Sydney Harbour Bridge, and the Brooklyn Bridge.