yes it is a polynomial function.
What is a polynomial? hmm well its a function that must satisfy the following conditions:
1.) n is a non negative number, meaning its equal to 0 or a positive number. n is the power, in that function the powers, are 4,2 and 1 they all satisfy this.
2.) The coefficients are real numbers. The coefficients here are 1, -1, 3 these are all real numbers so this is satisfied.
3.) the degree of the function is not 0. Degree of the function is the higest power there, so the degree of that function is 4. This is satisfied.