What can I do?My ex has fraudulently filed tax returns for the last 4 years claiming my son when I have had custody of him,she also acquired all his stimulus payments

[Critter-3]

All you can do is paper file all those returns when the IRS rejects them for the child already being claimed and let the IRS figure out what is what.  If you file the 2021 return then you will also get the CTC no matter what she did.

 There is a special rule in the case of divorced & separated (including never married) parents. When the non-custodial parent is claiming the child as a dependent/exemption/child tax credit; the custodial parent** is still allowed to claim the same child for Earned Income Credit, Head of Household filing status, and day care credit. This "splitting of the child" is not available to parents who lived together at any time during the last 6 months of the year; then only one of you can claim the child for any tax reasons. The tax benefits may not be split in any other manner. 

Note in particular that the non-custodial parent can never claim the Earned Income Credit, Head of Household filing status or the day care credit, based on that child, even when the custodial parent has released the exemption to him.

 So, it's good idea to let the other parent know that you will be claiming those items, as many first time divorced parents are not aware of this rule and may try to claim those items, which will cause the IRS to send out letters.

Ref: https://www.irs.gov/publications/p17#en_US_2017_publink1000170897

Scroll down to "Children of divorced or separated parents (or parents who live apart)"

** The IRS goes by physical custody, not legal custody, for who is the custodial parent.  Furthermore, for tax purposes, there is no such thing as joint custody, regardless of what your legal agreement says. The requirement, to be custodial parent, is that the child live with you MORE than 50% of the time. One of you has to be the custodial parent and the other the non-custodial parent. The IRS goes by physical custody, not legal custody. 

[Opus 17]

If you have physical custody more than half the nights of the year, and you do not give your ex a signed form 8332 dependent release, then you are the only person entitled to claim your child as a dependent. If you are blocked from e-filing, you must print your return and file it by mail. The IRS will review the duplicate dependent claim and award it to whichever parent has the best proof of where the child actually lived.

what you should have done for the past four years is printed your tax return and filed by mail, instead of deleting the child and e-filing. Now, you will have to go back and file amended tax returns for each of those past years.  TurboTax has revised the amending process.  In some cases, you may be able to amend a prior year from inside TurboTax online, if you used TurboTax online originally. For other years, you may need to download special amending software to your own computer.

At this point, you may amend your 2018, 2019, and 2020 tax returns, as long as you file the amended returns before April 15, 2022.  2017 is too old to amend.  You will likely also be able to file amended state tax returns to get the state benefits of claiming your child as a dependent.

https://ttlc.intuit.com/community/amending/help/how-do-i-amend-my-return/00/27439

https://turbotax.intuit.com/tax-tips/amend-return/how-to-file-an-amended-return-with-the-irs/L6kO691...

For 2021, the advice is the same. If you can’t e-file because your ex has claimed your child, do not remove the child from your tax return. Print your return, sign it, and mail it to the IRS.

when the IRS investigates your amended returns, they will be looking for proof of where the child actually physically lived during the years in question. A court order giving you physical custody will be helpful, but it may not be enough.  The IRS is especially interested in documents from unrelated third parties that show that the child lived in your house. This might include things such as a copy of the bus schedule showing that the child is picked up at your house five days a week for school, or letters from your child‘s doctor or school district that are sent to you because you are the custodial parent. You may even include photos from social media showing the child in your home throughout the year.  It may be difficult to prove for past years that you had actual physical custody, depending on whether you have saved the necessary documentation. The IRS will investigate both you and your ex, and will try to determine who has the best proof of where the child actually physically lived.

lastly, you might consider filing a form 14039-T identity theft affidavit on behalf of your child. You would explain on the form that your ex has been claiming your child as a dependent without your permission and without having physical custody using the child‘s Social Security number and other information. If the IRS excepts your claim of identity theft, they will issue the child and identity protection pin code. This is a six digit pin code that changes every year and is mailed to you by the IRS each December. A tax return that claims the child as a dependent cannot be e-filed without the proper pin code. This would prevent your ex from claiming the child in the future.