If $y{{dy} \over {dx}} = x\left[ {{{{y^2}} \over {{x^2}}} + {{\phi \left( {{{{y^2}} \over {{x^2}}}} \right)} \over {\phi '\left( {{{{y^2}} \over {{x^2}}}} \right)}}} \right]$, x > 0, $\phi$ > 0, and y(1) = $-$1, then $\phi \left( {{{{y^2}} \over 4}} \right)$ is equal to :