Electrical testing methods have inherent limitations. The two-electrode method is prone to noise from contact resistance due to polarization, while the four-electrode method reduces polarization but may disturb the soil structure during probe penetration, affecting measurement accuracy. Variations in penetration depth also alter current field attenuation, thereby influencing resistivity results. To address these issues, an indoor four-electrode penetration testing system was developed, using sandy and clayey soils as test materials. Calibration tests were conducted to evaluate the effects of saline concentration and voltage, and the influences of water content, dry density, and soil type on resistivity during penetration were analyzed. The results show that saline concentration and boundary conditions have negligible effects on electric field propagation. Voltage variation induces thermal and polarization effects that determine the calibration voltage. With increasing penetration depth, current attenuation weakens, and the resistivity of both soil types rises and stabilizes, following an exponential relationship with penetration depth—particularly under low water content and dry density due to the soil's high-resistivity nature. Finally, based on the experimental results, the minimum penetration threshold for four electrode resistivity testing of sand and cohesive soil materials was determined. The research results provide necessary data support and theoretical basis for the evaluation and application of electrical testing technology.