Bioexpress

Hey everyone, I got an email from one of the schools I applied to where they asked for a 4th rec letter from a PI that I worked with. The thing is that this PI lost their tenure, and no longer works as a faculty member at any school at the moment. Besides, I did not have an amicable relationship with this particular PI and would not trust them to write a favorable rec letter. How would I respond to the admissions office regarding their request? Any advice would be appreciated. Thanks!