Here is a some more info on the 4464C Letter:

If your return is pulled for review, you will most likely receive a letter from the IRS called a 4464c. If you get a letter from IRS, don’t panic and don’t ignore it: open your mail and read it.

There are a few variations on a theme but most letters 4464c likely start out something like this:

We have received your income tax return and are holding your refund until we complete a thorough review of your return. This review is part of an ongoing program the IRS conducts to ensure the accuracy of return information.

The letter should then indicate what the IRS may be concerned about. So the letter might say something like:

Your return was selected because we are reviewing one or more of the following:
– Income you reported on your return
– Income tax withholding amounts you reported on your return
– Claims for tax credits you made on your return
– Business income you reported on your return

You’ll then be instructed on what immediate steps to take, if any. It may well be that the letter simply says:

You are not required to do anything at this time. If you have not received your refund or been contacted by us within 60 days from the date of this letter, you may call us at the number provided above.

Hold onto that letter because you’re going to want to call the number on the letter within the stated time frame if your refund is still outstanding. You should not just rely on the “Where’s My Refund?” tool.

This is not the same as a proper examination or audit, it’s a review. If it escalates to an examination, you’ll be contacted by IRS to say what’s next – that usually involves providing additional documentation. It may be that you need to call in a tax professional to assist you at that point.