The Fourier cosine series of a function is given by : $$f\left ( x \right ) = \sum_{n = 0}^{\infty} f_{n} \cos nx$$ For $f\left ( x \right ) = \cos^{4} x$, the numerical value of $\left ( f_{4} + f_{5} \right )$ is __________. *(round off to three decimal places)*