1. First, you need to complete the tutorial task of Age of Empires 4. After completing the tutorial, you will enter the historical battle mode of the game. In this mode, you need to complete a series of campaign tasks, which will take you through different historical periods, from feudal times to industrial revolution.

2. Secondly, in the process of completing the historical battle mode, new civilizations and countries will be gradually unlocked. Among them, unlocking Japan requires the completion of a battle called Mongolian invasion.

3. After the Mongolian invasion campaign is finally completed, Japanese civilization will be unlocked. At this time, you can choose Japan to play in the random map mode of the game.