If it is busy they will load parties of 1, 2, or 3 with other parties of 1, 2 or 3. There is no way to guarantee your own car but you have a better chance during a low crowd time. They usually announce this while you are in line over the PA.
If you have 4, you will get your own car. Since they seat 3 per side, they will not make you sit with strangers on your side.