Final answer:
The bladder in a 4-year-old is located in the pelvic cavity, specifically in the lower part of the abdominopelvic regions of the ventral body cavity.
Step-by-step explanation:
The bladder in a 4-year-old is located in the pelvic cavity. The human body has two main cavities, the ventral cavity and the dorsal cavity, with the ventral cavity being at the front of the trunk. This ventral cavity is then subdivided into the thoracic cavity and the abdominopelvic cavity. As suggested by its name, the abdominopelvic cavity is further divided into the abdominal and pelvic cavities. The bladder, which is part of the urinary system, is positioned in the pelvic cavity, particularly in the inferior part of the abdominopelvic regions. This placement below the abdominal cavity ensures its protection by the bony structures of the pelvis.