Figuring out a vba code to use

You can use this code in the applicable worksheet:

Private Sub Worksheet_Change(ByVal Target As Range)

If Not Intersect(Target, Range("X4")) Then
    Application.EnableEvents = False
    Range("D4").ClearContents
    Application.EnableEvents = False
End If

End Sub

To insert the code, right click on the sheet tab, select View Code, and copy.