The IRS does use the W4 as guidance to see what should have been withheld for the year. It is unusual for the employee to request greater withholding and not have it be granted. It is usually the other way around, The employee will tell the employer to not take any withholding and the employer needs to find out why. Here is a link with a little information regarding the W4, IRS and Employer relationship. With your situation, the employer would not get in trouble as the IRS would see it as a human error. IRS, W4, Employer
