Sorry, but you don't get an additional $1,400 for the new baby. The stimulus payment that you received was an advance payment of the Recovery Rebate Credit on your 2021 tax return. But the stimulus payment was made in 2021, before you could have filed your 2021 tax return. So the IRS used your 2020 tax return as an estimate of what you would be entitled to on your 2021 tax return. In your case the estimate based on 2020 turned out to be on the mark. Based on your 2021 tax return you are entitled to $5,600, and that's exactly how much they paid you in advance. The fact that the estimate was based on different dependents doesn't matter. You're entitled to $5,600 for yourself, your spouse, and two dependents, and that's what you got. You don't get any more.
