  The structure of a Nuttall partition into sheets of some class of
four-sheeted Riemann surfaces is studied. The corresponding class of
multivalued analytic functions is a special class of algebraic functions of
fourth order generated by the function inverse to the Zhukovskii function. We
show that in this class of four-sheeted Riemann surfaces, the boundary between
the second and third sheets of the Nuttall partition of the Riemann surface, is
completely characterized in terms of an extremal problem posed on the
two-sheeted Riemann surface of the function $w$ defined by the equation
$w^2=z^2-1$. In particular, we show that in this class of functions the
boundary between the second and third sheets does not intersect both the
boundary between the first and second sheets and the boundary between the third
and fourth sheets.
