Over the past few years, technology has progressed and developed rapidly, where ad hoc networks have contributed a significant part to innovative development. There are four varieties of ad hoc networks such as mobile (MANET), vehicular (VANET), flying (FANET), and sea (SANET). Due to the variation of specifications, these four ad hoc networks have to turn into an alternative for providing connectivity in areas where infrastructure-based networks cannot be deployed. Therefore, this paper sets out a review of these four ad hoc networks, particularly focusing on the characteristics such as routing, density, and mobility. In addition, the variances between the four ad hoc networks are discussed with related works. This paper also outlines the challenges to be addressed in the deployments of these ad hoc networks.