Four-needle probes are widely used for the electrical characterization of biological tissues between 10Hz and 10MHz. The measurement accuracy depends strongly on the electrode/sample interface impedance in low frequency and on the parasitic capacitances of the measurement system in high frequency. Using a finite element model coupled with a deterministic optimization algorithm, it is shown that the geometry of the probe should be different at 10Hz and 10MHz to improve the performances of the measurement system.