As I recall, in NT4, the other fields are used in one of the examples;
Accounts(31) on the Line-Items form
where you select a product - it also populates the "price" as a default, based on the product selected.
Try that and see if it works. If it _does_ and you still don't come right, then I recommend emailing me an example of what you're doing (or posting it here).
cheers
Bruce