Best approach is to get rid of the 4 separate fields and put a related table of line items in place of the current set of 4 fields. This way, you can use a single relationship to look up the paper data for all items ordered.
Using separate order fields like this, you not only limit an order to 4 items, but must define separate relationships for each field to a different occurrence of the related table--which get's pretty messy.
You might take a look at this simple invoicing system to see how a related line items table could be implemented: http://fmforums.com/forum/showpost.php?post/309136/
This is exactly what I have been looking for! I knew there had to be a better way.
PhilMod - the link you posted is no longer valid and I am trying to get a better understanding of the conceptual framework you describe. Is there a more current link?