California's education data, at least as they currently are collected, managed, and made available, are not sufficient for understanding and addressing the needs of California's students. That's the conclusion of the first three reports in the series "California Education Policy, Student Data, and the Quest to Improve Student Progress," which seeks to shed light on the role and potential of student-level data systems in California (see "About This Series"). This fourth report finds that California lags behind many states that have already developed valuable ways to use cross-sector statewide longitudinal data systems (SLDSs) to improve policy and practice and to provide critical information to policymakers, educators, taxpayers, and students and their families. Fortunately, California has a range of options for developing an SLDS; there are numerous examples in other states that illustrate the possibilities. These data systems use a variety of structures and processes to link and manage student data across education systems to gain valuable information for improving policy and practice. [For Part I of this series, see ED574475; for Part II, see ED584700; and for Part III, see ED584699.]