In Chapter 4 of A Wrinkle in Time, the narrative delves deeper into the concept of the tesseract, which is pivotal for the children's journey through time and space. The chapter opens with Meg, Charles Wallace, and Calvin O'Keefe still reeling from their encounters with the celestial beings, Mrs. Whatsit, Mrs. Who, and Mrs. Which, who have introduced them to the idea of traveling through dimensions.

The chapter begins as the children gather in a tense atmosphere, filled with both excitement and apprehension about what lies ahead. They are eager yet uncertain regarding the journey they must undertake to rescue Meg's father, who is trapped in a dark and dangerous place. Mrs. Whatsit explains that the tesseract is not just a geometric concept but a means of traversing the universe that allows them to bypass the limitations of traditional travel, enabling them to move through time and space instantaneously.

The concept of a tesseract, often described as a 'wrinkle' in time, is illustrated through simple analogies that provide a clearer understanding for both the children and the readers. Mrs. Whatsit emphasizes that just as a two-dimensional being cannot comprehend the third dimension, the children must let go of their conventional understanding of space to grasp the higher dimensions they are about to enter.

As the children absorb this information, they are filled with both wonder and fear. The dialogue reflects their varying levels of comprehension; Charles Wallace, with his extraordinary intelligence, begins to grasp the implications of the tesseract more quickly than Meg and Calvin. Despite this, Meg expresses her insecurities, feeling overshadowed by her brother's brilliance and questioning her own abilities. Calvin, on the other hand, attempts to reassure Meg, demonstrating his growing bond with her and his protective instincts.

After a moment of preparation, the children are guided by the celestial beings to begin their journey. They experience a sense of disorientation as they enter the tesseract, which is depicted as a swirling, chaotic passage that bends the very fabric of reality. This journey is not just a physical transition; it symbolizes their emotional and psychological growth as they step into the unknown, confronting their fears and uncertainties.

The chapter concludes with the children landing on an unfamiliar planet, setting the stage for their next challenges. They are filled with a mix of trepidation and determination, aware that they are on a mission that could alter the course of their lives. This chapter effectively captures the essence of adventure and the transformative power of love and friendship, laying the groundwork for their forthcoming encounters with the forces of evil and the mysteries of the universe.