This paper presents computations of a four tidal turbine array, with a row of three upstream devices and a downstream turbine. The studied configuration is based on the layout initially proposed in the framework of the NEPTHYD project. The simulations were carried out with a three-dimensional unsteady Lagrangian Vortex software. A synthetic eddy method is used to take into account the ambient turbulence encountered in tidal energetic sites. The loads perceived by the turbines are estimated with a lifting line approach, recently added in the software. To study the interaction between the upstream and downstream turbines, numerical velocity maps, wake lines, unsteady velocity variations obtained with numerical probes as well as fluctuations of power and thrust coefficients are presented.