The spectroscopic electrical characterization of biological tissues between 10 Hz and 10 MHz is generally based on four-electrode impedance measurement. An array of four aligned needle-electrodes is commonly used for the probe. The electrode/sample interface impedances and the parasitic capacitances of the system induce measurement inaccuracies respectively in the low and high parts of the studied frequency range. These inaccuracies cannot be removed by any usual calibration method but can be minimized by modifying the geometry of the probe. Using a 3D finite element model of the probe coupled with a deterministic optimization algorithm, it is shown that the configuration of the electrodes should be different at 10 Hz and 10 MHz to improve the performances of the measurement system.