Do you mean a car two that's a two seater coupe with two occasional seats ?
ie the occasional seats are too small for any real comfort so you can't claim its a four seater.
2+2 is just 2+2, because it isn't a question, its just a statement. If you want to know 2+2=, or 2+2?, then its 4, but otherwise its 2+2 until there is an equals sign.