There are exceptions to the statute of limitations (in the Texas Civil Practice and Remedies Code) but in general, a claim you knew about that is more than four (4) years old is subject to the defense of being barred by passage of time. The claim can be pursued but is subject to that defense from the statute of limitations.
Note also, it does not yet feel settled whether a counter-claim arises when a time-barred lawsuit is filed.
Also, I am not at this time the lawyer for you or the new owner or anyone else related to the business and property this story refers to and I am making assumptions about the facts. I am easy enough to find to talk this through or make referral.
Further, the written agreement with the new owner can control your rights and responsibilities regarding what she wants you to pay. After that, there is the straight up business deal aspect of this in addition to the legal rights and responsibilities of the parties.
The foregoing comments are provided for informational purposes only, and without a fee agreement and fees paid there is no attorney-client relationship being made and no legal services will be provided now or later.