Sub Comma()
  Dim aRng As Range
  Set aRng = ActiveDocument.Range
  With aRng.Find
    .ClearFormatting
    .Text = "[0-9]{4,}"
    .Wrap = wdFindStop
    .MatchWildcards = True
    Do While .Execute = True
      If aRng.Text Like "20??" Then
        aRng.HighlightColorIndex = wdBrightGreen
      Else
        aRng.Text = Format(aRng.Text, "#,##0")
      End If
      aRng.Collapse Direction:=wdCollapseEnd
    Loop
  End With
End Sub