  By using the free field worldsheet realization described by Gaberdiel and
Gopakumar recently, we construct the nontrivial lowest generators of the higher
spin superalgebra $hs(2,2|4)$. They consist of cubic terms between the
bilinears of ambitwistor-like fields. We also obtain the worldsheet description
for the findings of Sezgin and Sundell twenty years ago given by the familiar
oscillator construction. The first order poles of the operator product
expansions (OPEs), between the conformal weight-$1$ generators of Lie
superalgebra $PSU(2,2|4)$ and the above conformal weight-$3$ generators of
$hs(2,2|4)$, are determined explicitly and the additional generators appear in
the worldsheet theory.
