a mettre dans le module de ta feuille
Private Sub Worksheet_Change(ByVal Target As Range)
Application.EnableEvents = False
If Target.Row Mod 4 = 0 Then
Select Case Target.Column
Case 1
If Target.Offset(0, 3) = "" Then
Else
MsgBox ("NON")
Target.Value = ""
End If
Case 4
If Target.Offset(0, -3) = "" Then
Else
MsgBox ("NON")
Target.Value = ""
End If
End Select
End If
Application.EnableEvents = True
End Sub