While leaving a thick layer of leaves under trees and shrubs can provide a nice winter mulch, 4 feet is too much of a good thing! About a foot is all that is needed to reap the benefits. When the pile gets so high as to cover crowns and trunks, it can potentially lead to rot as the leaves get wet with winter weather. Also, such a high pile provides habitat for undesirable critters such as rats. You are right … he should be taking some of the leaves away.
While leaving a thick layer of leaves under trees and shrubs can provide a nice winter mulch, 4 feet is too much of a good thing! About a foot is all that is needed to reap the benefits. When the pile gets so high as to cover crowns and trunks, it can potentially lead to rot as the leaves get wet with winter weather. Also, such a high pile provides habitat for undesirable critters such as rats. You are right … he should be taking some of the leaves away.