At present, none of the wireless access or mobile cellular network technologies provides ubiquitous coverage and the complete set of services required by the users. Therefore, the next generation of wireless networks i.e. 4th Generation (4G) network aims at the integration of these distinct yet complementary networks to satisfy user requirements and to provide ubiquitous coverage, anywhere and anytime. This integration of heterogeneous networks requires efficient and fast switching from one network to another, which is possible through seamless vertical handover. This paper presents a proactive, seamless vertical handover algorithm that caters for user preferences, mobile station velocity, network conditions and application requirements. It selects a candidate network for handover which is stable and can provide necessary services required by the applications. The proposed algorithm improves handover performance by minimizing signaling overhead and delay.