The Fourth Generation (4G) mobile networks are based on Long Term Evolution (LTE) technologies and are being deployed worldwide, whilst research on further evolution towards the Fifth Generation (5G) has been recently initiated. 5G will be featured with advanced network infrastructure sharing capabilities among different operators. Therefore, an open source implementation of 4G/5G networks with this capability is crucial to enable an early research in this area. The main contribution of this paper is the design and implementation of such a 4G/5G open source testbed to investigate multi-operator infrastructure sharing capabilities executed in virtual architectures. The proposed design and implementation enables the virtualization and sharing of some of the components of the LTE architecture. A testbed has been implemented and validated with intensive empirical experiments conducted to validate the suitability of virtualizing LTE components in virtual infrastructures, i.e. infrastructures with multi-tenancy sharing capabilities. The impact of the proposed technologies can lead to a significant saving of both capital and operational costs for mobile telecommunication operators.