Effective, effectual, efficient, efficacious all mean producing or capable of producing a result or results, but they are not freely interchangeable in idiomatic use.
Effective emphasizes the actual production of an effect or the power to produce a given effect.
Effectual suggests the accomplishment of a desired result or the fulfillment of a purpose or intention, so that the term frequently becomes synonymous with decisive or final and looks backward after the event.
Efficient may apply to what is actively operative and producing a result and then comes close to operant in meaning. More often it suggests an acting or a capacity or potential for action or use in such a manner as to minimize the loss or waste of energy in effecting, producing, or functioning.
As used of human beings with this denotation efficient suggests the exercise of such qualities as skill, pains, and vigilance and often becomes synonymous with capable and competent.
Efficacious implies the possession of the quality or virtue that gives a thing the potency or power that makes it effective.