Not sure what you want but I guess you would like to get the colum number of the last filled cell in the first row. Then you might try this
Range("m4").Value = Cells(1, Columns.Count).End(xlToLeft).Column
In order to use your approach you would need the following function
Function colLetter(col As Long) As String
colLetter = Split(Columns(col).Address(, 0), ":")(0)
End Function
and then
Range("m4").Value = Range(colLetter(Columns.Count) & "1").End(xlToLeft).Column
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