The person who bought my last house did an FHA loan. I'd heard horror stories about their inspections (like having to scrape / repaint the trim on a detached garage because it was flaking in a few places), but the guy I dealt with was cool. He asked to see the city permits & sign offs for all of the recent work I had done and disclosed to the seller, did a home inspector type walkthrough, and then at the end asked me what price the buyer and I agreed on. His appraisal was $100 over the amount I told him.
If you got the official ding for mold on the report, you will have to somehow remediate it to appease them. The test alone can't be $4K, so spend the hundred bucks or so to find out what it is. If it is mold but not a hazard to the occupants, they might just lower their appraisal amount to adjust for its presence. If they red-flag the property or the appraisal comes in too low, then you can contact the buyer and negotiate who will pay for the remediation. Unfortunately, if it is mold and this sale falls through you'll now have to disclose that to anyone else interested in the property.