What is the coefficient of 
in 
$$\\ \small{\text{
\begin{array}{l}
\text{set } $ 1+x+x^2+x^3 = (1+x)(1+x^2) $
\text{ we have }$\left[(1+x)(1+x^2)+x^4 \right]^4 $\\
\text{now we expand} $\\$ $[(1+x)(1+x^2)]^4
\underbrace{
+ 4*[(1+x)(1+x^2)]^3(\textcolor[rgb]{1,0,0}{x^4})+6*[(1+x)(1+x^2)]^2(\textcolor[rgb]{1,0,0}{x^4})^2+4*[(1+x)(1+x^2)](\textcolor[rgb]{1,0,0}{x^4})^3+ (\textcolor[rgb]{1,0,0}{x^4})^4
}_{\text{We drop this part, because all terms here is multiplied with $x^4 $ and are $> x^3$ } }
$ \\
\text{we analyse }$ (1+x)^4(1+x^2)^4 $$\\$\text{we expand } $ (1+\textcolor[rgb]{0,0,1}{4x} +6x^2+ \textcolor[rgb]{1,0,0}{4x^3} +x^4) ( \textcolor[rgb]{1,0,0}{1} +\textcolor[rgb]{0,0,1}{4x^2} +6x^4+4x^6+x^8 ) $ $\\$\text{the terms with } $x^3$ are $\textcolor[rgb]{0,0,1}{4x}*\textcolor[rgb]{0,0,1}{4x^2} $ and $\textcolor[rgb]{1,0,0}{4x^3}*\textcolor[rgb]{1,0,0}{1} $ $\\$the sum is $\textcolor[rgb]{0,0,1}{16x^3} + \textcolor[rgb]{1,0,0}{4x^3} = 20x^3$ $\\$the coefficient of $x^3$ is $20$
\end{array}
}}$$
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+26387 
