The fourth in a series, this publication presents a statistical picture of Illinois Public Libraries during the 1990-1991 fiscal year. Its purpose is to provide librarians and trustees with statistics that can be compared to those of other libraries of similar size and environment to determine whether the library is above or below the average for its system or population group, to justify library actions and budgets, to plan for library service, and to evaluate the library's progress. The information was derived from the 1990-91 Illinois Public Library Annual Report forms. Three statistical views of Illinois public libraries are presented. Section I, Illinois Public Library Statistics Listings, provides data about library operations, use, resources, and finances for each public library submitting an Illinois Public Library Annual Report of Fiscal Year 1990-91. Section II, Public Libraries Grouped by Size of Population Served, describes statistical averages and percentiles for libraries providing direct service to populations of different sizes. Section III, Public Libraries Grouped by System, reports statistical averages and percentiles for libraries providing direct service with membership in one of the state's 18 library systems. An executive summary provides state-level statistics and compares library services, income, and expenditures with those in other states. Appended are: (1) Library Locations and Legal Names Cross-Reference; (2) Definitions and Acronyms; and (3) Illinois Public Library Annual Report Form 1990-1991. (KRN)