Well, I strongly suspect any body fat reading lower than 5%. That is about as low as human beings can get without expiring. I have heard people claim 0% which means just really lean. They are blowing smoke. Physiologically, it is impossible. Your cell membranes are made of fat (phospholipids). If you had a 0% body fat, you would just fall apart because you would have no cell membranes.
None of the scales is very accurate. The skin fold is OK for comparative purposes but not absolute.
I suspect the only truly accurate way is to grind up the person in a blender and render off the fat. But that is a tad bit hard on people so most people decline doing things the truly accurate way. Where has dedication gone anyway?
Your reading of 3.9% is freakishly lean. Body builders hit low body fat levels but just briefly and they say it is like being almost dead. I suppose it is possible but it is not likely.