I covered the subject of coilovers in depth not too long ago, and in truth there are/were
only 4 companies offering them that can fit the ForFour; usually the Brabus only though;
they are usually for the Colt CZT/CZC Turbo......with one or maybe 2 exceptions.
1/ FK...who make 2 models......both have poor write up's on-line. (Think they fit the Brab only)
2/ BC...made for the Colt CZT....need a little modding, but are not that bad. (Only fit the Brab)
Don't think they have rebound adjustment...
3/ Bilstein's...made for the Colt CZT...but JJ has a set on his Brab. (Only fit the Brab)
No rebound adjustment.
4/ KW's....made for ForFours and Colts. They fit all models....the 'crown jewels' of coilovers.
They have rebound adjustment.
However, the biggest 'hurdle' will always be the price....
FK's start from about £500, BC and Bilsteins are circa £650+, and KW's top the magic £1k !