The value of \zeta(4) with fourier series Given a function as follows :
$f(x) = \pi^2 - x ^2$ on $|x|<\pi$ and $f(x+2\pi)=f(x)$
I did find its Fourier expansion
$f(x)=\frac{2}{3}\pi^2$+$\sum_{n \geq 1}\frac{4}{n^2}(-1)^{n+1}cosx$
And by putting $x=\pi$, I got the zeta of 2 , $\zeta(2) = \sum_{n \geq 1}\frac{1}{n^2} = \frac{\pi^2}{6}$
I d like to find the zeta of 4,$\zeta(4) = \frac{\pi^4}{90}$ with fourier expansion. If possible, what is the method ? 
If this function $f(x)$ is Not proper, What shlould i use another function? Please, advise me.
I do guess that it may be used Square Error

$f(x) = \pi^4-x^4$, $f(x+2 \pi)=f(x)$
$$f(x) = \sum_{n=0}^{\infty} a_n \cos{n x} $$
$$a_0 = \frac1{2 \pi} \int_{-\pi}^{\pi} dx \, (\pi^4-x^4) = \frac{4 \pi^4}{5}$$
$$a_n = \frac1{\pi} \int_{-\pi}^{\pi} dx \, (\pi^4-x^4) \cos{n x} = (-1)^{n+1} \frac{8 (n^2 \pi^2-6)}{n^4}$$
Thus,
$$f(x) = \frac{4 \pi^4}{5} - 8 \sum_{n=1}^{\infty} (-1)^{n} \frac{n^2 \pi^2-6}{n^4} \cos{n x}$$
$$f(\pi) = 0 \implies \frac{4 \pi^4}{5} - 8 \pi^2 \sum_{n=1}^{\infty} \frac1{n^2} + 48 \sum_{n=1}^{\infty} \frac1{n^4} = 0$$
Using
$$\sum_{n=1}^{\infty} \frac1{n^2} = \frac{\pi^2}{6} $$
We may deduce that
$$\sum_{n=1}^{\infty} \frac1{n^4} = \frac{\pi^4}{90} $$
