Theorem about iterative roots from "Contributions to general algebra 6"

Question

$ F(x)={a}_4x^4+{a}^3x^3+{a}_2x^2+({a}_1+1)x+{a}_0 $ has a polynomial as a quadratic iterative root if and only if there is $\gamma$ -a cubic root of ${a}_4$ such, that $4\widetilde{a}_4A_4=A_2^2+2\gamma A_2$ and $A_3=-1$, where \begin{equation} A_m=\sum_{k=0}^m \Big(\frac{-\widetilde{a}_3}{4\widetilde{a}_4}\Big)^k {{4-m+k}\choose k}\widetilde{a}_{4-m+k} \quad\quad m\in \{ 0,1,2,3,4\} \end{equation} I need to see a proof for this theorem, and i even know i which book i can find it. It should be in
Schweizer, B., Sklar, A., 1988. Invariants and equivalence classes of polynomials under linear conjugacy. Contributions to General Algebra 6, Holder-Pichler-Tempsky, Vienna. pp. 253–257.
But there is no way i can get access to that book. If anyone could help with the proof, or can post one from the book itself i would really apriciate.