this book constitutes the joint refereed proceedings of the 4th international workshop on communication technologies for vehicles/trains, nets4cars 2012 and nets4trains 2012, held in vilnius, lithuania, in april 2012. the 4 full papers of the rail track and 11 full papers of the road track presented were carefully reviewed and selected from numerous submissions. they provide an overview over latest technologies and research in the field of intra- and inter-vehicle communications (protocols and standards), mobility and traffic models (models, methodologies, and techniques), testing, and applications.