Many history curricula are set up for sequential, civilization-centered study. But given the choice, we still prefer chronological study. A child who learns chronologically is much more likely to remember the dates and purposes of these monuments, because he's encountered them three times in three different cultures all during the same time period. These connections aid the child's memory. Chronological study also makes the teaching of history easier, since civilizations are always visiting each other and borrowing from each other.
Chronological study opens the child's eyes to the relationships between different civilizations.
Finally, doing a chronological study improves a child's geography. More importantly, it may never occur to the student that someone might travel from the Nile Valley to Greece and back again, carrying customs and ideas with them. Given the value of chronological study, then, the four-year repetitive pattern makes sense. This is why our Great Books list for high school covers four years. Beginning ancient history in grade 7 or 8 means that a student encounters some of the most technically difficult literature on the list at a younger age, when many students are not yet quite mature enough to put the necessary effort into reading it.