How do admissions committees handle a missing “extra” letter of recommendation? If a student has sent 4 letters, one more than required, and the 4th letter is missing, what will happen?

Will the committee contact the applicant if his application is strong enough?
Will they ignore the letter and decide based on the other 3? Will they delay the review till they have the 4th letter?

I know this varies between schools but how it's usually handled?
If the required number of letters have been received, then most likely the candidate’s application will be reviewed normally, with no penalty attached for not having the fourth letter. If a required letter was missing, then things get a little dicier, depending on whether there are too many applications and if it’s a rolling or fixed admissions cycle.