Using the particle-in-cell simulation code MAGIC, a vacuum microelectronics tetrode with a planar emitter has been simulated. The electron beam streamlines have been evaluated, which show that the upper gate focuses the electron beam. The current distribution between gate and anode has been calculated and the effect of the lower gate voltage and the emitter size on emitted current and gate capture current has been investigated. Simulation results are presented for a single vertical diamond-film field-emission tetrode with the anode biased at 300 V and spaced 50 mu m from the cathode. Simulation results indicated that the radius of the electron beam at the anode is about 20 mu m for a microtriode, but only about 6 mu m for the microtetrode. The results show that gate capture current should not be negligible in most cases, and can be ignored only if the emitter radius is less than half of the gate aperture radius. Then the percentage of gate capture current to emitted current is less than 0.1%.