The number of turning points of a polynomial function is related to its degree. Specifically, a polynomial of degree \( n \) can have at most \( n - 1 \) turning points.
In this case, since the polynomial has four turning points, the least possible degree \( n \) can be determined by the inequality:
\[ n - 1 \geq 4 \]
Solving this gives:
\[ n \geq 5 \]
Therefore, the least possible degree of the polynomial is 5.
So the answer is:
5