man, that is a rough spot. 4 year old meat stored in butchers paper is no bueno. My uncle pulls that all the time. Since he raises his own cattle, it is probably safe to say that they are grass fed, which will have a slightly different taste than the corn fed beef people are used to eating, and that is if it is not freezer burned.
What you do depends on your relationship with this supervisor and how candid he allows you to be with him.
If your relationship is good with him, all you have to do is mention "hey do you know that the package of meat you gave me is over 4 years old?". It is possible that he didn't look at the date and thought he was giving you something newer. Hopefully, he will realize his error and will either hook you up with something more suitable or spring for a decent brisket.
If your relationship is not open with the guy, or if he is a tyrant that doesn't allow his subordinates to voice concerns candidly, then just buy another brisket on your own and cook both. You simply cannot let that garbage represent your cooking. But I would still cook the meat he gave you. Just don't do anything that would make you appear that you are trying to show him up. Cut off all visible evidence of freezerburn and apply a good coat of red wine vinegar the night before. That will tame some of the off flavors, but will not fix it completely. I would definitely foil with a good amount of bbq sauce to mask the off flavors.
Primo Oval XL, BPS drum, Weber gasser