This paper presents the architecture and performance evaluation of a virtualized wide-area "4G" cellular wireless network. Specifically, it addresses the challenges of virtualization of resources in a cellular base station to enable shared use by multiple independent slice users (experimenters or mobile virtual network operators), each with possibly distinct flow types and network layer protocols. The proposed virtual basestation architecture is based on an external substrate which uses a layer-2 switched datapath, and an arbitrated control path to the WiMAX basestation. The framework implements virtualization of base station's radio resources to achieve isolation between multiple virtual networks. An algorithm for weighted fair sharing among multiple slices based on an airtime fairness metric has been implemented for the first release. Preliminary experimental results from the virtual basestation prototype are given, demonstrating mobile network performance, isolation across slices with different flow types, and custom flow scheduling capabilities.