OWL is a logic-based ontology language standard designed to promote interoperability, particularly in the context of the (Semantic) Web. The standard has encouraged the development of numerous OWL reasoning systems, and such systems are already key components of many applications. The 4th OWL Reasoner Evaluation Competition (ORE 2015) compares and evaluates 14 OWL reasoning systems on a data set containing a wide range of ontologies from the web, obtained through a standard web crawl and the Google Custom Search API, and a snapshot from the well known BioPortal repository. Some user submitted ontologies spice up the corpus with particularly relevant and difficult test cases. The ontologies in the test set are binned by profiles to enable the participation of reasoners specialised on the EL and DL profiles of OWL. The evaluation is performed in three different categories that cover the important tasks of ontology classification, consistency checking, and ontology realisation (i.e., the computation of entailed types of individuals).