As far as I know, the whole point of the ForFour is to persuade people to switch over to the Smart brand without them having to resort to driving the smaller variety (now called the ForTwo), to generate income for future unique products.
I remember reading an article in a magazine with a convoy of Smart cars (ForTwos) going through Death Valley for hot weather testing..the locals looked somewhat amazed by how small these cars really are, and how well they were standing up..and to think they have a 0.8L engine!