The "creditor" suing you can be made to prove in court that you made a payment & that the SOL has not elapsed. They can be made to come up with payment date, payment method (ie check with ck. #), etc.

They can also be asked in court to prove that they bought the debt from the original creditor & that the original creditor is out of the picture - that they are the ones the money is now owed to & not original creditor.

You are correct that the SOL on this debt is 4 years in Tx.