The 4th International Computer Science Symposium in Russia (CSR 2009) was held August 18-23,2009 in Novosibirsk,Russia, hosted by the Sobolev Institute of Mathematics and Novosibirsk State University. It was the fourth event in the series of regular international meetings, following CSR 2006 in St. Petersburg, CSR 2007 in Ekaterinburg, and CSR 2008 in Moscow. The opening lecture was given by Andrei Voronkov, and four other invited plenary lectures were given by Sergei Odintsov, Wolfgang Thomas, Nikolai Vereshchagin, and Hongseok Yang. This volume contains all the accepted papers and some of the abstracts of the invited speakers. The scope of the proposed topics for the symposium was quite broad and covered basically all areas of computer science. We received 66 papers in total, and the Program Committee selected 29. Yandex provided the Best Student Paper Awards; the recepients of these awards were selected by the Program Committee: - Dmitry Itsykson, "Structural complexity of AvgBPP" - Yuri Pritykin and Julya Ulyashkina, "Aperiodicity measure for in?nite sequences." The reviewing processwasorganizedusing the EasyChairconferencesystem, created by Andrei Voronkov.We are grateful to our sponsors: - Russian Foundation for Basic Research - Yandex (the largest Russian Internet portal providing key Web services). We also thank the group of local organizers and in particular Pavel Salimov.
