Jetzt läuft es perfekt. Danke vielmals....

So siehts jetzt aus.

Sub TransferDetailedEstimate()
Dim clave As String

clave = "test"

        If MsgBox("This command will owerwrite everything in this Sheet." & "Do you really want to transfer?", vbYesNo + vbQuestion) = vbYes Then
With Application
    If (Join(.Transpose(Sheets("Detailed Estimate").Range("C4:C2000")), vbNullString) = Join(.Transpose(Sheets("Detailed Forecast").Range("C4:C2000")), vbNullString)) = True Then
    Application.ScreenUpdating = False
    Sheets("Detailed Forecast").Unprotect clave
    Sheets("Detailed Estimate").Range("D4:D2000").Copy
    Sheets("Detailed Forecast").Range("D4").PasteSpecial Paste:=xlPasteValues
    Sheets("Detailed Estimate").Range("L4:L2000").Copy
    Sheets("Detailed Forecast").Range("E4").PasteSpecial Paste:=xlPasteValues
    Application.CutCopyMode = False
    Sheets("Detailed Forecast").Range("D4").Select
    Sheets("Detailed Forecast").Protect clave, AllowDeletingRows:=True, AllowFormattingColumns:=True, AllowFormattingCells:=True, AllowFiltering:=True, UserInterfaceOnly:=True
     Else: MsgBox "Number of lines is not the same. Please check and correct the number of lines.", vbCritical
    
    End If

Application.ScreenUpdating = True
    
End With
End If

End Sub