Clearing the contents of a cell based on updates values of other cells

Private Sub Worksheet_Change(ByVal Target As Range)

    If Target.Cells.Count > 1 Then Exit Sub
    
    If Not Intersect(Target, Range("A1:A3")) Is Nothing Then
        Range("A4").ClearContents
    End If

End Sub

The code must go into the worksheet code module.
Right-click the tab of the target worksheet, choose View Code then copy and paste the above code into the code pane.
Close the VB Editor.

Anytime a change is made to cell A1:A3, cell A4 will be cleared.