There is no 4th stimulus.
If someone else is accessing your IRS online account, it sounds like you need to file a report of identity theft and get the account closed and redirected to you. You might try the Taxpayer Advocate if you can't get help with the IRS directly. https://www.irs.gov/taxpayer-advocate
You can also file an identity theft affidavit by mail with the IRS on form 14039-T.
https://www.irs.gov/newsroom/when-to-file-a-form-14039-identity-theft-affidavit
The IRS has other links on that page to assist with identity theft.
