I hope that helps. I'm not sure I understand all the nuances of meaning set out there - and this is from the second edition, not "New Fowler" so it comes from a time before the blossoming of management consultancy.Efficacious applies only to things (especially now to medicines) used for a purpose, and means sure to have, or usually having, the desired effect.
Efficient applies to agents or their action or to instruments etc, and means capable of producing the desired effect, competent or equal to the task.
Effectual applies to action apart from the agent, and means not falling short of the complete effect aimed at.
Effective applies to the thing done or its doer as such, and means having a high degree of effect.