A manufacturer purchased and installed a shrink-wrap machine 4 years ago at a cost of $4,000. A new machine is now needed, and one is available for $7,000 less a $1,000 trade-in allowance for the old machine. The market value of the old machine without trade-in on a new model is $500. Which of the four dollar values above is a sunk cost that is irrelevant in a pre-tax engineering economic analysis