Toolz,

You mean if there are multiple cartridges in your database which are
"low", and you want the message box to include all of them?  Hmmm, well
that is theoretically possible, but I would suggest that a MsgBox is not
really the appropriate approach with this.  I would try it like this
instead...
Make a query based on your table, with a Criteria <4 in the Quantity field.
Make a continuous view Form based on this query.
On the After Update event of the existing form (not the Quantity
control), put code like this...
 Private Sub Form_AfterUpdate()
   If DCount("*","YourQuery") > 0 Then
      DoCmd.OpenForm "Your New Form"
   End If
 End Sub

For the record, to do it with the MsgBox idea, the code would be
something like this...
 Private Sub Quantity_Exit(Cancel As Integer)
   Dim rst as DAO.Recordset
   Dim MessageText As String
   Me.Dirty = False
   Set rst = CurrentDb.OpenRecordset("SELECT [Type of Cartridge] FROM
YourTable WHERE Quantity < 4")
   Do Until rst.EOF
       MessageText = MessageText & rst![Type of Cartridge] & vbCrLf
       rst.MoveNext
   Loop
   rst.Close
   If Len(MessageText) Then
         MsgBox "Ink is low:" & vbCrLf & MessageText
   End If
   Set rst = Nothing
 End Sub