Start by drawing force vectors on charge q. You will notice that if the force vectors from both of the -Q charges are combined, the resultant will line up with the force vector from the +4Q charge.
So, use pythagorean theorem to find the resultant of the two -Q forces using Coulomb's equation F = k*q1*q2/r^2, and then use this result to subtract from the force of the +4Q charge.
The final answer will look similar this:
F = (K|4Q∣∣q∣ / 2L^2) − (sqrt(2)*K|Q∣∣q∣∣ / L^2)