4,200 is the 3rd Stimulus payment. It was 1400 for each person. So Married with 1 dependent would be 1400x3=4,200. Did you already get the Stimulus payment they sent out last year starting March 2021? The IRS has records that show you were sent the 3rd stimulus payment so they removed the 4,200 Recovery Rebate Credit.
Do you have an amount on 1040 line 30? You might have tried to claim it again. Line 30 is only if you didn't get the full amount or qualify for more.
Did you get IRS letter 6475 saying what they sent you? If you are married you should each get letter 6475 for half.
Double check your bank statements and check your IRS account to see what they sent you. Check both IRS accounts if married.
Your Online Account | Internal Revenue Service
If the IRS thinks they sent it to you but you didn't get it request a trace for a missing third Stimulus payment
https://www.irs.gov/newsroom/questions-and-answers-about-the-third-economic-impact-payment-topic-j-p...Your Online Account | Internal Revenue Service
