\[y_{3} = 4;\ \ y_{6} = 500:\]
\[\left\{ \begin{matrix} y_{1}q^{2} = 4\ \ \ \ \\ y_{1}q^{5} = 500 \\ \end{matrix} \right.\ \ \ \ \ |\ :\]
\[\frac{1}{q^{3}} = \frac{1}{125}\]
\[q^{3} = 125\]
\[q = 5.\]
\[y_{1} = \frac{4}{q^{2}} = \frac{4}{25}.\]