No. The third expression x^-3 + 4x is not a polynomial because the exponent is -3. Polynomials can have exponents but only 0, 1, 2, 3, and the rest of the positive integers are allowed. They can have constants and variables that can be combined by the four basic operations except division by a variable like 1/x or like the third expression x^-3 + 4x which is also 1/x^3 + 4x. The other three given expressions are polynomials.