<cr>
A lot depends on the paint, color, and what you are spraying over. I would probably buy 3 quarts but consider these 2 points.

1) The 4th quart is cheap, in comparison with the first 3.
2) You only need to worry about the color matching on the cab and clip. If you ran out of paint while painting the interior and had to buy extra paint, a slight mismatched shade difference wouldn't make a difference.