Hey I work at one of the big four! I'll hire you if you march your age out!
Okay, I don't actually have that much power and I'm not even in the accounting department (work in technology) but to be serious, an internship isn't going to be your only ticket into a career at one of the big four. Most people who work there didn't come through an internship. Yes, the internship can help you get your foot in the door if you do a good job, but don't think of it as your one shot. In fact, if your resume was good enough to get offered an internship, then it will probably be enough to get an interview once next year rolls around.
When people are deciding to march or not my spiel is always since I graduated college I've been teaching as a hobby ever since. Having marched drum corps is what allows me to do that and excel at it. It's definitely a lifelong skill. So if you haven't marched before and teaching is something you are interested in, I would definitely suggest it.
If, however, you see yourself really throwing yourself fully into your career soon, then maybe that internship would be worth it. You can also consider marching a DCA corps. They will give you a great experience while allowing you to pursue your internship at the same time.
Hope this helps!