\textbf{169.} Electric charges $+q$, $q_1 = +q$ and $q_2 = q_3 = +3q$ are placed at the vertices of an equilateral triangle, and the electric charge $q_4 = -q$ is placed at the centroid of this triangle, kept fixed. If the force exerted by $q_1$ on $q_4$ is $0.5$ newtons, what is the resultant of the forces exerted on $q_4$ in newtons?
(1) zero \hfill (2) $0.5$ \hfill (3) $1$ \hfill (4) $0.5\sqrt{3}$