Charges a and b in the figure are equal. each charge exerts a force on the other of magnitude

f. suppose the charge of b is increased by a factor of 4, but everything else is unchanged.

The formula used to determine the force between two charges is

F = kQ1Q2/d2

The first has a and b with equal charges

F = k q q / d2

F = kq2/d2

In the second scenario, the charge of b is increased by 4

F = k q 4q / d2

F = 4kq2/d2

This means that increasing the charge by 4 also increases the force between them by 4.