Why didn't the elders decide to allow birthmothers to have four baby instead of three in the giver?

The decision to limit birthmothers to three babies instead of four was likely made by the Elders as a way to control population growth within the community. By restricting births to three per birthmother, they could manage and maintain a stable population size without putting excessive strain on resources. Additionally, keeping the birth limit at three may have been a way to maintain order and conformity within the society's strict rules and regulations.