You are not legally allowed to cash out your ex's 401k. . 401k plans are individual retirement accounts that are setup and administrated through the employer.. If you are divorced and part of the settlement is a portion of your ex's 401k, you need a legal document, called a QDRO. (Qualified Domestic Rations Order) in order to access any of your ex's account. You will need a lawyer.. If your ex "promised" you a portion of his/her 401k, then they would have to withdraw it from their own account. . Be aware that the government punishes those who withdraw money from a 401k with a 10% penalty fee on withdrawals. Also, you will owe federal and state income tax on any money that is withdrawn from a 401k. Example, if you take out $10,000 and you are in the 20% tax bracket, you will pay $2000 in income taxes, a penalty of $1,000 (10%) and you will only be left with $7,000.