What did the titles which divided the story to 4 parts indicate?

Ask Again, Yes has 22 chapters plus a prologue. The chapters are divided into the sections Gillam, Queens, Two by Two and Muster. Muster, the concluding section, is also the longest, covering six chapters. The other sections contain four to five chapters. Many chapters begin with time stamps, including the Prologue, which begins in July of 1973. The novel spans a great deal of time, from the 1970s to the 2000s. It covers the time before Peter and Kate were born, their childhood and then a great deal of their adulthood together.