In three-dimensional (3D) TIs, topological surface states have been successfully investigated by surface-sensitive techniques such as angle-resolved photoemission spectroscopy. However, finding evidence of transport through the surface states by regular transport measurements remains a challenge owing to the presence of parallel bulk conducting channels that usually dominate the transport properties. We explored these surface states by measuring the four-probe resistance in a special geometry, putting the voltage and current contacts at unusual positions. Our results show a crossover from bulk to surface conduction below 20 K in Bi2Te2Se. Besides this, the magnetoconductance provides clear evidence of weak anti-localization, which also indicates a strong contribution of topological surface states.