No citizen shall be denied, because of his failure to comply with any test or device, the right to vote in any Federal, State, or local election conducted in any State or political subdivision of a State.

As used in this section, the term “test or device” means any requirement that a person as a prerequisite for voting or registration for voting (1) demonstrate the ability to read, write, understand, or interpret any matter, (2) demonstrate any educational achievement or his knowledge of any particular subject, (3) possess good moral character, or (4) prove his qualifications by the voucher of registered voters or members of any other class.