As a parent, you should tell them there isn't anything under the bed. If they are still complaining, then comfort them and show them the truth that there are no monsters under the bed whatsoever.
Well the piaget theory is based on the devlope of intelligence and logic - so to a 4 year old they are not capable of understanding your explanation of "no such thing as monsters." So what would be the best decision is to not tell them it's not real but not feed into their imagination. They are to young to understand if it's not true at their age.
