Excel Macro: If the final characters in each row of a column is "something", then

I'm trying to concatenate two cells if certain conditions are met in one of the cells.

Specifically: If the final characters in the cells of column D = " XX " then concatenate. I've done something similar to below and get an error each time.

Sub concatenate()
Last = Cells(Rows.Count, 4).End(xlUp).Row

  For i = Last To 1 Step -1

    If Right(Cells(i, 4), 4) = " XX " Then
    'do some stuff to concatenate'

    End If

  Next i

End Sub

Any help is greatly appreciated.