If you wanted to use a formula in a helper column, you could do something like:
=left(a1,3)&"."&right(a1,1)
maybe better to check to see if something "nice" was entered:
=IF(LEN(A1)<>4,A1,LEFT(A1,3)&"."&RIGHT(A1,1))
If you want to "format" the same cell, you'll have to use a macro:
Right click on the worksheet that needs this stuff.
select view code and paste this in:
Option Explicit
Private Sub Worksheet_Change(ByVal Target As Range)
If Target.Cells.Count > 1 Then Exit Sub
If Intersect(Target, Me.Range("a:a")) Is Nothing Then Exit Sub
If Len(Target.Value) <> 4 Then Exit Sub
On Error GoTo errHandler:
With Target
Application.EnableEvents = False
.Value = UCase(Left(.Value, 3)) & "." & Right(.Value, 1)
End With
errHandler:
Application.EnableEvents = True
End Sub
I used column A, so adjust that range as required.
A couple of links that may help:
Chip Pearson's site:
http://www.cpearson.com/excel/events.htm
or
David McRitchie's site:
http://www.mvps.org/dmcritchie/excel/event.htm
If you're new to macros, you may want to read David McRitchie's intro at:
http://www.mvps.org/dmcritchie/excel/getstarted.htm