On stable polynomials of degrees $2,3,4$

Let $q$ be a prime power. We construct stable polynomials of the form $b^{m-1}(x+a)^m+c(x+a)+d$ over a finite field $\mathbb{F}_{q}$ for $m=2,3,4$ by Capelli's lemma. When $m=3$ and $q$ is even, we confirm the conjecture of Ahmadi and Monsef-Shokri [2] that the polynomial $f(x) = x^3 + x^2 + 1$ is stable over $\mathbb{F}_{2}$. Moreover, when $m=2$ and $q\equiv 1\pmod{4}$, we improve a lower bound of the number of quadratic stable polynomials by Goméz-Pérez and Nicolás [4].