Actually the IRS has revised its stance on when employers have to send in a W-4:
"Q1: In the past, as an employer, I was required to submit all Forms W-4 that claimed complete exemption from withholding (when $200 or more in weekly wages were regularly expected) or claimed more than 10 allowances. What Forms W-4 do I now have to submit to the IRS?
A1: Employers are no longer required to routinely submit Forms W-4 to the IRS. However, in certain circumstances, the IRS may direct you to submit copies of Forms W-4 for certain employees in order to ensure that the employees have adequate withholding. You are now required to submit the Forms W-4 to IRS only if directed to do so in a written notice or pursuant to specified criteria set forth in future published guidance."
see https://www.irs.gov/businesses/small-businesses-self-employed/withholding-compliance-questions-and-answers for further information.
Now it is possible that some states, mine included, still have a requirement to send in the tax withholding form if it is above a certain #....