On the construction of ${\cal N}=4$ SYM effective action beyond leading   low-energy approximation
  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. 
A.T. Banin, N.G. Pletnev