The Credit is for Qualified Retirement Savings Contributions, not just having $4000 in your IRA. You have until April 18, 2023 to make qualified contribution for tax year 2022.
If you made a $4000 IRA contribution, earlier in 2022, that counts for the credit. Edited, per dmertz's reply: $2000 is the limit for each spouse. You would have to contribute $2000 to your IRA and $2000 to your spouse's 401k (or IRA) to get the maximum credit . See line 6 of form 8880.
The Retirement Savings Contributions Credit (“Saver’s credit”) is very limited and is for low income people. See the table on Form 8880: https://www.irs.gov/pub/irs-pdf/f8880.pdf
You, or your spouse, must not have taken money out of a retirement account in the last three years. See line 2 on form 8880.
It is not a refundable credit. You must have a tax liability to take the credit against.
