There's no direct property to do that, BUT...
You can check the lenght of the text in the EditBox of the ComboBox and if it is 4 then don't allow the user to enter anything else:

Put this on ComboBox KeyPress event:

Code:
Private Sub Combo1_KeyPress(KeyAscii As Integer)
    If Len(Combo1.Text) >= 4 And KeyAscii <> vbKeyBack Then
        KeyAscii = 0
    End If
End Sub
------------------

Serge

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