I would go for “4 meters and a tiny bit”. When driving in a thick fog that limits vision to 4 meters, a sane driver interprets “don’t see anything” as “there’s at least 4 meters of room”, not as “5 meters” or “10 meters”.

That also makes sense if you interpret “sensor says 3” not as “obstacle is 3 meters away”, but as “there’s 3 meters of room”.

But then, I didn’t try to see what works better. I find the result surprising, though.