4) \(\frac{1}{\frac{1}{21}+\frac{1}{28}}\)

\[\frac{1}{21}+\frac{1}{28}=\frac{4}{84}+\frac{3}{84}=\frac{7}{84}=\frac{1}{12}.\]

Следовательно, \[\frac{1}{\frac{1}{21}+\frac{1}{28}}=\frac{1}{\frac{1}{12}}=12.\]

Ответ: \(12\).