This volume contains the invited paper, the reviewed research papers, the discussants' comments on the latter, and the workshop memoranda of the fourth IFIP WG 8. 1 International Working Conference on "e;Information System Concepts"e;, with the subtitle "e;An Integrated Discipline Emerging"e; (ISC04). It was held in Leiden, The Netherlands, 20-22 September 1999. The previous three ISCO conferences were subtitled "e;An In-depth Analysis"e;, "e;Improving the Understanding"e;, and "e;Towards a Consolidation of Views"e;. Their proceedings were published in 1989, 1992, and 1995, respectively. The ISCO conferences were instigated by the former Task Group FRISCO (an acronym for "e;FRamework of Information System Concepts"e;), charged by the IFIP Working Group 8. 1 with the task of proposing a conceptual framework for the information system field. Its report, entitled "e;A Framework of Information System Concepts"e; (the "e;FRISCO Report"e; for short), is available on the World Wide Web via the address: http://www. wi. leidenuniv. nl/~verrynstlfrisco. html or may be downloaded directly as a condensed Word-6 file from: ftp://ftp. leidenuniv. nl/pub/rullfri-full. zip The FRISCO Report forms a significant contribution to the long-lasting quest of our community towards developing a scientific outlook on the field of information systems. Clarifying the varied nature of many diverging views - some of which may not easily be reconciled - the report does propose a coherent, consistent and partially formalised framework of concepts.