For any of us to help you, we need to know more about the ForFour you have,
as with those details we will then be able to 'narrow' down the possible answers
or suggestions to those that may be relevant, as each model has differences, ok.
So...is it a:-
(1) a 454, or a 453 ForFour
(2) what size is the engine
(3) is it petrol or diesel
(4) is it a manual or an AMT
(5) and though not strictly relevant..what model (ie, Pure, Pulse, Passion, Cool Style, or Brabus)
Cheers