  A problem of the hidden ${\cal N}=2$ supersymmetry deformation for
next-to-leading terms in the effective action for ${\cal N}=4$ SYM theory is
discussed. Using formulation of the theory in ${\cal N}=2$ harmonic superspace
and exploring the on-shell hidden ${\cal N}=2$ supersymmetry of ${\cal N}=4$
SYM theory, we construct the appropriate hypermultiplet-depending contributions
for $F^6$ term in the Schwinger-De Witt expansion of the effective action. The
procedure involves deformed hidden ${\cal N}=2$ supersymmetry and allows one to
obtain self-consistently the correct ${\cal N}=4$ supersymmetric functional
containing $F^{6}$ among the component fields.
