We develop a novel bi-harmonic $$ \mathcal{N} $$
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= 4 superspace formulation of the $$ \mathcal{N} $$
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= 4 supersymmetric Yang-Mills theory (SYM) in four dimensions. In this approach, the $$ \mathcal{N} $$
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= 4 SYM superfield constraints are solved in terms of on-shell $$ \mathcal{N} $$
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= 2 harmonic superfields. Such an approach provides a convenient tool of constructing the manifestly $$ \mathcal{N} $$
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= 4 supersymmetric invariants and further rewriting them in $$ \mathcal{N} $$
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= 2 harmonic superspace. In particular, we present $$ \mathcal{N} $$
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= 4 superfield form of the leading term in the $$ \mathcal{N} $$
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= 4 SYM effective action which was known previously in $$ \mathcal{N} $$
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= 2 superspace formulation.