There are some rules an equation must follow to be considered a polynomial.
Polynomials in fourth order follows the following form:
Where "x" is our variable and a,b,c,d and e are scalars.
Our expression looks to follow the same form but as we can see bellow:
There are a negative exponent. And one of the rules it must follow to be a polynomial is "do not have negative exponents".
And because of that, our euqation is not a polynomial.