This text constitutes the refereed proceedings of the 4th Italian Conference on Algorithms and Complexity, CIAC 2000, held in Rome, Italy, in March 2000. The 21 revised full papers presented were carefully reviewed and selected from 41 submissions, also included are four invited survey papers. Among the topics addressed are: combinatorial optimization; graph algorithms; graph computations; complexity theory; diagram design; approximation; scheduling; sorting; computational geometry; searching; and pattern matching.