I've never been at the big 4 (or 5) but I have worked for a Fortune 100 company.
By the time you get to the last interview, they are not looking for a technical fit - they already think you fit. They may ask you a technical question, but they are probably more interested in how well you articulate your ideas than how much you know.
Always sell yourself. I have personally made the mistake of being too low key. Think about what you can do for them - what do they need that you have?
An old boss of mine taught me the three things you look for in a Candidate;
Can they do it?
This is probably screened out by now - they are wasting a directors time if they don't already know you can do it.
Will they do it?
This speaks to drive, determination, ambition.
Will they fit in?
Find out something about the corporate culture to see if there is a match between it and you. It could be your approach to teamwork, work-life balance, all kinds of issues. Find out what thet are like and identify some commonalities.
James