The Fourth International Conference on Algorithmic Decision Theory has been held on September 27 - 30, 2015 in Lexington, Kentucky (USA) and organized by Judy Goldsmith (conference chair) and Toby Walsh (program chair). In addition to preferences in reasoning and decision making, the conference covered algorithmic challenges to modern decision support and automation, uncertainty and robustness in decision making, multi-criteria decision analysis and optimization, collective decision making, decision-theoretic artificial intelligence, and machine learning and knowledge extraction for decision support. The conference accepted submissions for proceedings which have been published as a volume of the Lecture Notes in Artificial Intelligence series as well as submissions without proceedings that are published elsewhere. ADT 2015 has been co-located with the 13th International Conference on Logic Programming and Nonmonotonic Reasoning LPNMR 2015. Lexington is situated in the Bluegrass region and known for horse races, basketball, and Bourbon. The conference proceedings have been published as Volume 9346 of the Lecture Notes in Artificial Intelligence series.