4) $$(m^6 - \frac{2}{3}n^5)(m^6 + \frac{2}{3}n^5) = (m^6)^2 - (\frac{2}{3}n^5)^2 = m^{12} - \frac{4}{9}n^{10}$$
Ответ: $$m^{12} - \frac{4}{9}n^{10}$$