(1) The obligation is secured by bonds of any state of the United States or bonds of any county, city, school district, port district or other public body in the United States;
(2) The principal amount of the obligation is not more than 90 percent of the market value of the bonds that secure the obligation;
(3) The bonds are payable from ad valorem taxes; and
(4) The bonds are rated in one of the four highest grades by a recognized investment service organization that has been engaged regularly and continuously for a period of not less than 10 years in rating state and municipal bonds. [1997 c.631 §155]