The rear electric windows are part of the Comfort Pack that includes electric and heated wing mirrors, armrest, light/rain sensor and all 4 electric windows with one-touch function (on the fronts at least)
Spec levels are common in the Smart range, there were Pure, Pulse Passion and brabus. The Brabus package can be had on any forfour, including leather seats, but there are still differences from the proper Brabus, the logo in the instruments, steering wheel, seats and headrests, center front bodypanel, etc.
Towards the end of the forfour life some new model levels came out, Blackbasic, Purestyle, Coolstyle, Sportstyle...a last effort to sell off stocks I suppose, they were mostly basic trim level models
So no, you don't have to buy a Brabus to have rear electric windows, any model could have them if the first buyer chose the Comfort Package...which was an option on all other models except Brabus which got it as standard of course...being the top of the range model, it had all the optional extras.