Adding cars to the game doesn't have to be that difficult to do imo.
They can make a combination of the Sims 2 and the Sims 3 cars easily. They can give us the animations for opening the doors, getting in, listen to radio, woohoo etc. on the driveway, but as soon as Sims want to ''drive away'' the car teleports onto the road and drives off. (If it makes sense in the world where they are in. I.e. in Windenburg there are parts where it's pedestrian only, so they wouldn't drive off, but just teleport away.)
This pretty much solves the ''not attached to a road'' problem.
And when Sims are arriving at their destination, they can either have a new parking lot object where cars automatically pop onto, or they can just ''put them in their inventory'' and when they are going back home again, or to a different lot, they can just pop it out and go. Not the most 'realistic' way, but well, it is the Sims 4 we are talking about.