Policy makers in California intend to provide free preschool to all 4-year-olds solely within public schools by 2026, becoming the nation's second largest single pre-K program in the United States after Head Start. This initiative builds on the state's existing Transitional Kindergarten (TK) option that has served a modest share of 4-year-olds since 2010. Tracing the historical growth in TK enrollments, we find that just 30, mostly urban school districts, enrolled two-fifths of all children served by 2020, responding to funding incentives and displaying stronger organizational capacity. Meanwhile, one-third of California's nearly one thousand districts enrolled fewer than 12 TK children. Black, white, and Asian children remained disproportionally under-enrolled as a share of their respective populations, as enrollments climbed past 90,000 children prior to the COVID-19 pandemic. Identifying factors that may explain widely differing gains in TK enrollment, merging education and local census data, we find the suburbs began to catch-up with cities in serving additional 4-year-olds, as well as districts offering school choice (e.g., charter schools). We discuss implications for other nations attempting to rapidly expand preschool, including the inequities that may inadvertently arise.