It's possible that the children lost their eligibility because they did not meet the "Sought to Acquire" requirement. It seems you haven't submitted DS-260 for the children even after 4 years from the PD becoming current.
For family-preference categories, visa applicants (including derivative beneficiaries) must seek to acquire LPR status within one year of visa availability, to benefit from CSPA. Refer to https://fam.state.gov/fam/09FAM/09FAM050201.html
I am thinking that only the first two children were included in the original I-130 petition filed by the petitioner for her brother (in the respective section about beneficiary's family). The third child was omitted by mistake and the fourth child was not born. The priority date became current four years ago. When the DS260 was completed two months ago, the beneficiary listed the names of all children. There is a “Sought to Acquire” requirement Under the Child Status Protection Act. Which means beneficiaries should apply for the visa within one year once the PD becomes current. If the application was not made within that one year, they forfeit the claim. The third and fourth child did not lose the claim because they were only added on the DS260 recently.
