4) $$\sqrt{-b^{21}c^{26}} = \sqrt{-b^{20} \cdot b \cdot c^{26}} = \sqrt{b^{20} \cdot c^{26} \cdot (-b)} = b^{10}c^{13}\sqrt{-b}$$
Ответ: $$b^{10}c^{13}\sqrt{-b}$$