In the wireless communication, a latest generation is Fourth Generation (4G). A 4G system will be able to provide a comprehensive IP solution where voice, data and streamed multimedia can be given to users on an anytime, anywhere basis, and at higher data rates than previous generations. DoS (Denial of Service) attack is an attempt in which an attacker makes a network resource unavailable to the licensed user. In this paper the authors focus on the DoS attack of CRN (Cognitive Radio Network) using 4G communication. Cognitive radio network is nothing but a network in which two users can access the same spectrum band. These users are called licensed and un-licensed user.