When a force moves an object, work is done. The amount of work done is equal to the product of force and the distance through which the force is applied. Therefore, in this case, the work done by a 5N force moving a block 4 m can be calculated as follows:
Work = Force x Distance
Work = 5 N x 4 m
Work = 20 Joules
Therefore, the work done by a 5N force moving a block 4 m is 20 Joules.