1) Because the force at a distance d with charges q1 and q2 is

The force at half that distance will be

2) The force at twice the distance will be

3) The force when q1 is halved will be

The force when q1 is doubled will be

4) Because the electrostatic Coulomb force depends on the product of the two charges, changing one charge by a factor will have the same effect as changing the other charge by the same factor. However, adding to one charge (say, increasing its value by 4 coulombs) may have drastically different results, depending on which charge you change. Note also that if the two charges have the same sign (are both positive or both negative), then the force between them is positive; this corresponds to a repulsive force. If the two charges have opposite signs, then the force between them is negative; this corresponds to an attractive force.