we will use f=ma to solve this problem. the velocity of the drop before contact is 30m/s. according to your description, the drop goes from 30m/s to 0m/s when it hits the head, and this happens in a distance of one radius, ie 1mm.
1) use kinematics equations to solve for the acceleration given v0=30, vf=0, and d=1.
2) use f=ma, given mass (4mg) and acceleration found in part (1) to find the force required to cause such an acceleration.
if you do not know where to go with the kinematics equations, repost!