\(\tau= pEsin \theta\)
\(4= q \times \ell \times E \times \sin 30^{\circ}\)
\(4= q \times 2 \times 10^{-2} \times 2 \times 10^5 \times \frac{1}{2}\)
\(q =2 \times 10^{-3}\)
\(q =2\,mC\)
(Take $\frac{1}{4 \pi \varepsilon_{0}}=9 \times 10^{9} Nm ^{2} C ^{-2}$ )