Here's what ChatGPT 4o mini (free) answered to me to the same question:
No, Paul Edwin Zimmer did not write a fourth novel in the Dark Border series. The trilogy consists of "The Dark Border," "The Dark Border: The Return," and "The Dark Border: The Reckoning." After these, he focused on other projects and did not continue the series.
No, Paul Edwin Zimmer did not write a fourth novel in the Dark Border series. The trilogy consists of "The Dark Border," "The Dark Border: The Return," and "The Dark Border: The Reckoning." After these, he focused on other projects and did not continue the series.