After you file

You must mean the third stimulus amount for $1400 per person.   There has not been a fourth stimulus check.   Your letter means the IRS believes you received the 3rd stimulus, but you tried to claim it as a recovery rebate credit on your 2021 return.   You need to check your bank statements for deposits of $1400 per person after mid-March of 2021, and you need to check your online account with the IRS to see what  is shown there.  Do not count on your memory.    If you truly did not receive that stimulus amount but the IRS says it was issued you can have it traced.

if your refund was short by $1400 per person---it is likely to be a mix up regarding the 3rd EIP payment.    If you received the 3rd stimulus check of $1400 per person in 2021 then you got letter 6475 from the IRS about it.   You needed to enter the amount you received.   If you already received that money then line 30 on your Form 1040 should have been blank.

https://www.irs.gov/individuals/understanding-your-cp13-notice

LOTS of people have made mistakes this year with the recovery rebate credit by "forgetting" that they received the $1400 per person stimulus checks last year.  They answered no to the questions for the  recovery recovery rebate credit and put the $1400 per person on line 30 of their returns.  But you cannot get it twice, so the IRS removes it when it is there by mistake.  The IRS sent letter 6475 showing that you received the 3rd stimulus check. 

It is very important that if you filed a joint return—-the IRS sent letter 6475 to EACH spouse.  You had to enter the total amount received.   Many people misunderstood and entered only the amount from one of the letters.

Try checking your online account with the IRS (and if you filed a joint return—check for BOTH of you)

https://www.irs.gov/payments/your-online-account

Does the IRS show that you received a $1400 per person stimulus or EIP check in 2021?   Did you receive it?   When you were asked on your tax return did you say no?   Was there $1400 per person on line 30 of your tax return?

If the IRS says you got $1400  per person last year but you did not receive it you can have it traced.

LOST THIRD STIMULUS CHECK

https://www.irs.gov/newsroom/questions-and-answers-about-the-third-round-economic-impact-payment-top...

If you need to trace a payment that the IRS says has been issued to you, mail or fax a completed IRS Form 3911, Taxpayer Statement Regarding Refund.