Section 19J. Notwithstanding the provisions of sections nineteen G and nineteen H, a city or town which accepts this section may compensate a clerk of a city or town who also serves as a member of its board of registrars of voters, in addition to any compensation to which such clerk is entitled as city or town clerk, a sum not less than fifty dollars but not to exceed one hundred dollars, and, if the number of registered voters exceeds one thousand, an additional sum not less than fifty dollars but not to exceed one hundred dollars for each additional thousand of registered voters or major fraction thereof; provided, however, that the total payment for such service as a member of said board shall not exceed four thousand dollars in any year.