Final answer:
The formal agreement between bondholders and the issuer regarding the terms of the debt is called an indenture, which is option 4.
Step-by-step explanation:
The formal agreement between bondholders and the issuer as to the terms of the debt is known as an indenture. An indenture outlines the specific promises made to bondholders, including the interest rate, the repayment schedule, and any other conditions or covenants associated with the bond. This is different from a covenant, which is a specific term within an indenture, a prospectus that provides the details of an investment offering for sale to the public, or a debenture, which is a type of bond that is not secured by physical assets or collateral.