The degree of a polynomial is defined as the highest power of the variable present in the expression.

(i) In the polynomial $2x^{2}$ - 5x + 3, the highest power of the variable x is 2. Therefore, its degree is 2.

(ii) In the polynomial $y^{3}$ + 2y - 1, the highest power of the variable y is 3. Therefore, its degree is 3.

(iii) The number -9 is a constant polynomial. It can be written as -$9x^{0}$, where the power of the variable is 0. Thus, its degree is 0.

(iv) In the polynomial 4z - 3, the highest power of the variable z is 1. Therefore, its degree is 1.