Sec. 4. EXAMINATION, APPROVAL AND REGISTRATION OF BONDS.  Prior to delivery thereof, all bonds authorized to be issued hereunder and the record relating to their issuance shall be submitted to the Attorney General of Texas for examination;  and if he finds that they have been issued in accordance with the Constitution and this Act, and that they will be binding special obligations of the District authorizing their issuance, he shall approve them, and thereupon they shall be registered by the Comptroller of Public Accounts of the State of Texas, and after such approval and registration they shall be incontestable.