multiple work book sum

[K-Ching]

Sub SUM_WBs()

    Dim FileNameXls As Variant, i As Integer, wb As Workbook
    
    'ChDir = "C:\Temp\"      'Default directory
    
    FileNameXls = Application.GetOpenFilename(filefilter:="Excel Files, *.xl*", MultiSelect:=True)
    If Not IsArray(FileNameXls) Then Exit Sub    ' User cancelled
    
    Application.ScreenUpdating = False
    
    For i = LBound(FileNameXls) To UBound(FileNameXls)
    
        Set wb = Workbooks.Open(FileNameXls(i))
        wb.Sheets(1).Range("D4:S21").Copy
        ThisWorkbook.Sheets(1).Range("D4:S21").PasteSpecial Paste:=xlPasteValues, Operation:=xlAdd, SkipBlanks:=True, Transpose:=False
        Application.CutCopyMode = False
        wb.Close SaveChanges:=False
        
    Next i
    
    Application.ScreenUpdating = True
    
End Sub

So here is the situation copying workbook i have it but what i want is to fill in the blank.
Exemple: workbook 1 has part numbers and workbook 2 the same and i want to sum them in workbook total. as you can see i can not only copy paste.Attachment 292807Attachment 292808Attachment 292809