The unity feedback system given by

\[
\mathrm{G}(s)=\frac{4}{s\left(s^{2}+a s+2 bight)} ; a \text { and } b \text { are positive constants. }
\]

is limitedly stable and oscillates with frequency \(4 \mathrm{rad} / \mathrm{sec}\). Find \(a\) and \(b\).