This paper presents a simple algebraic representation of the discount function as a constrained linear combination of four exponential functions. By restricting its three term structure coefficient within a three dimensional simplex constraint set, the exponential-quadrinomial models guarantees the term structure to have a positive discount function, yield function, and forward function over the entire maturity range. The models leads to a simple formula that represents the present value of any bond, by a constrained linear combination of four hyphotetical presents values of the bond, plus a tax adjustment. The formula maps the market term structure underlying the observable bonds into the simplex by a simple ordinary least squares regression.