The second and third dielectric virial coefficients of 4He have been measured by the symmetrical gas-expansion technique of Buckingham, Cole and Sutter for six isotherms between 3,00 K and 17,68 K. The effects of helium adsorption are important, and are considered in detail. No temperature dependence of the second coefficient is seen at a level of δb=+/-0,008 cm3 mol-1, and average values of the second and third coefficients are b = 0,006 (10) cm3 mol-1 and c = -5 (4) cm6 mol-2, respectively. The uncertainty for b is relatively large because it has a systematic dependence on c: reanalysis of all the existing data shows that a more probable value of the third coefficient is c = -2 (1) cm6 mol-2, which when combined with our data gives b (3 K - 18 K) = -0,001 (4) cm-3 mol-1. The very small value suggests that theoretical calculations of the excess polarizability of the helium diatom need to be reconsidered. The small value and the small uncertainty of b indicate that future work to improve experimental values of the second density virial coefficient of 4He for this temperature range may without ambiguity make use of the methods of dielectric constant gas thermometry.