This book constitutes the refereed proceedings of the 4th International Conference on Rough Sets and Current Trends in Computing, RSCTC 2004, held in Uppsala, Sweden in June 2004. The 45 revised full papers and 60 revised short papers presented together with 3 invited papers and a bibliographic project report were carefully reviewed and selected from 248 submissions. The papers are organized in topical sections on theory, logic and rough sets, granular computing, rough and fuzzy relations, foundations of data mining, incomplete information systems, interestingness, multiagents and information systems, fuzzy logic and modeling, rough classification, rough sets and probabilities, variable precision rough sets models, spatial reasoning, reduction, rule induction, rough sets and neural networks, clustering, data mining, image and signal recognition, information retrieval, decision support, adaptaive and optimization methods, bioinformatics, and medical applications.