In this paper, we propose an innovative solution for positioning determination in 4G wireless networks by introducing the ad-coop positioning system (ACPS). The ACPS is supported by a hybrid cellular ad-hoc architecture, where the cellular network has a centralized control over the ad-hoc connections among pairs of mobiles. Specifically, P2P communications are exploited in a mesh fashion within cellular-established clusters for cooperation-aided localization purposes (from that, the word ad-coop is derived). The major contribution of this work is the design of the data fusion method for the ACPS, which appropriately combines and weights the long- and short-range location information in a non-linear least square (NLLS) minimization procedure. The numerical results show that the ACPS enhances the location estimation accuracy with respect to conventional hybrid positioning techniques in cellular networks.