Answer is: concentration of hydroxide ions [OH⁻] is 1.047·10⁻¹⁰ M.
pH = 4.02.
pH = -log[H⁺].
[H⁺] = 10∧(-pH).
[H⁺] = 10∧(-4.02).
[H⁺] = 9.54·10⁻⁵ M; equilibrium concentration of hydrogen cations.
[H⁺] · [OH⁻] = 1·10⁻¹⁴ M².
[OH⁻] = 1·10⁻¹⁴ M² ÷ 9.54·10⁻⁵ M.
[OH⁻] = 1.047·10⁻¹⁰ M; equilibriom concentration of hydroxide anions.