Security and usability plays an important role in the success in today's computer, mobile, and technology based system. In these systems, security gets higher priority for high security measures ignoring the fact that the system may lead to an unusable one. However, everyday technology users need a system that is usable and also safe. In this paper, we propose a novel approach for usable security for mobile system i.e. a mobile system that will be usable and also secured. Traditional mobile device authentication techniques do not consider the combination of user biometrics, environmental information, and information provided by the sensors within the pervasive system. In this paper, we present a four way fusion of user authentication techniques for efficient usable security on mobile devices. In this approach user is not required to remember any alphanumeric password. The location traces, gait pattern, emotion of user and context of an image is used as metric for authentication. This system silently does authentication using location traces and gait pattern of its user. If the silent authentication fails then system authenticates its users based on user's emotion sequence and an image context previously stored by the user. Our proposed fusion based authentication technique can be useful for numerous real life scenarios where mobile device security is important.