offers need to be based on market value. if there are none out there, or none at lower prices, good luck getting anything off. you don't get to get a "deal" just because you think that's the way car sales are done...
all the people on here are the first to talk about how the dealership model/car sales are goofy, yet you tell people to offer 4 thousand off a car based on *zero* knowledge of the market, the car, or anything? come on guys... that is a part of the problem.
now, they aren't wrong that the worst they can say is "no"... but if anyone comes in my store offering that much off, they are told nicely, "no, we don't vary much in pricing on used cars". is there a comparable vehicle you found for this price of 20k or what is it that has made you decide on this number? if it is just because you want a discount, unlikely it will happen. again, feel free to ask, but understand the market before just throwing numbers out.