Here's the question:
This almost went the direction OP hoped it would, or rather it would have had suspenders not found their way into the question. Everything went downhill from there.
Case in point, this joker:
I came from a regional firm where we mostly wore business casual. I showed up to my first day at Deloitte without suspenders on and I forgot my green visor. I was immediately fired.
You guys are dicks. You clearly forgot suspenders
Let's not forget:
At the Big 4 office itself, you'll likely find people in business casual. It can vary by region, but generally this means a button down and slacks (no tie or jacket). Big 4 workers themselves often dress to their client's office's code, and may have to wear formal attire every day they are on that particular engagement. If you're visiting a Big 4 office for the first time, however, I'd recommend staying on the side of formality. This would be an outfit involving red suspenders, a pinstriped shirt, blue pants, a bowtie, and potentially a green visor if you'll be meeting with a partner.
Plus for our friends across the pond:
Braces (if you're in the UK)
Now, was all that trolling necessary? Probably not. But come on, let them have a moment, they are so few and far between. Not to mention it's likely the firm will totally go over this if you are hired — and by "go over" I mean "hammer it into your brain over and over and over until you freaking understand how to dress like an adult with a real job" — so it's not even the appropriate question to ask a bunch of clowns on the Internet.
It's also entirely possible that OP was the one doing the trolling. I mean it is the Internet, after all. One never knows.