What to do without underwear on a 4 day trip?

Wash them in the sink before you sleep, let them dry as much as possible until the morning, if they are not dry by then, iron them a bit and they will be dry.

If you can't get a hold of an iron, put them in front of the A/C or the heater, do not block the airway totally, just let the air pass through the boxers and it will make them dry in no time.

I'm assuming you're at the 2015 BC Football High School Camp (Session II) - (quick Google search). Funny they did not list underwear in the Things to Bring section. On this page it mentions a Camp Store. Talk to someone at the store to see if they can pick some up for you. If money is an issue, have them contact someone (parent) who can figure that part out. Also, you could contact someone using one of the many numbers or email provided in the Contact Information section. This goal of this approach is to get some new skivvies versus the sink and iron approach. Also, check around with some of the other players to see if they forgot stuff too. Having a list of things for several people may help.

Inspired by the same-day delivery answer, I remembered there's a service called "Magic" where you basically text them what you want and they quote a price:

We have trained operators standing by 24/7 to answer every one of your requests. Send us a text message, and we'll get you what you want. We'll order what you need from the appropriate service (e.g. DoorDash, Instacart, Postmates, etc.), and deal with them so you just automatically get what you want, like magic...

and from the FAQ:

Q: What kinds of things can I use Magic for?

Anything you want. As long as it's not illegal. Seriously. Just try it.

It might be useful if you have no money or credit cards as someone else could do it for you.

I've no idea if they're any good, I've always wanted to try it when I'm in the states but never have. I'm mostly posting because I think it would be awesome to ask a random service to find and deliver you underwear.

You're at football camp? Someone else brought extra underwear. Barter for them.

If you have a couple of shorts with different length with you, you can use the shortest pair as underwear. Or even better, use swimwear - at least that dries quicker after washing them.

There's also: - Ask a friend - Ask a teacher - Ask a random stranger

Personally I'd just wash them, you might need to improvise with soap or shampoo. It's what I do on business trips: take 2-3 pairs and wash the most recently used pair every evening. As an added benefit this also means I don't have to wash a mountain of dirty clothes when I come back after 4 weeks of travel. Just make absolutely sure that socks are completely dry before wearing them.