Age of Empires IV almost always has the same number of ages available in a playthrough. Obviously, there are some civilizations that don't play by the rules when it comes to the number of ages they progress through in Age of Empires IV. However, most will go through four.
The four ages in Age of Empires IV are the Dark Age, the Feudal Age, the Castle Age, and Imperial Age. The majority of the games you play will see you start at Age I and then work your way up.
The ages of Age of Empires IV
The Dark Age is considered Age I in Age of Empires IV. This is where you will typically start a game and build your civilization from. The basics of this age will see you exploring, scouting, and hunting.
The next age can then be reached by creating a specific Landmark that coincides with the civilization you are playing. This first one offers very little in terms of technology and military might.
The Feudal Age is where you will see your Age of Empires IV civilization truly start to grow. You gain access to many more building types and your military can become quite the formidable force.
It is an important period of economic development that will usually set the tone for the rest of your civilization's existence. Depending on the strategy you employ, the Feudal Age can make or break your civilization's future.
The third age in Age of Empires IV is the Castle Age. This is where the game starts to get intense. Your civilization's specific Castle will now be available to build and more powerful units for your military can be created.
The Castle Age is where you will truly see the expansion of your empire. Multiple town centers become available and a host of technologies can be studied to improve your status.
The final age you will progress to in Age of Empires IV is the Imperial Age. Your civilization will now reach the highest might of its military force and be able to research the best technologies available.
In this age, you can build a Wonder for your civilization. Some good examples of Wonders would be the Italians' Cathedral of St. Lawrence or the Celts' Rock of Cashel.