At which position would the electric force be the greatest?

Point 4

Explanation:

As per the properties of electric field lines we know that density of field lines represent the intensity of electric field.

So here as we can see that the distribution of electric field lines is given in this figure which shows that lines are closer to each other at region marked as #4

So this closed field lines at this region shows that the electric field strength is maximum at this point.

So here maximum electric field is at #4

now we know that force on an electric charge is given as

F = qE

so force will be maximum at #4 where electric field is maximum

find the sum of the two horizontal vectors and the sum of the two vertical vectors

isoceles right angled triangle