Perhaps
[VBA]Sub test()
Call InsertTextEveryN(Range("A1"), 4, " ")
End Sub
Sub InsertTextEveryN(aRange As Range, everyN As Long, insertedText As String)
Dim LOfText As Long
Dim i As Long
LOfText = Len(CStr((aRange.Cells(1, 1).Value)))
For i = Application.Floor(LOfText, everyN) + 1 To 2 Step -everyN
aRange.Cells(1,1).Characters(i, 0).Insert (insertedText)
Next i
End Sub
[/VBA]