In quite simple terms the answer to fitting to the tailgate is 'NO'.
The upper edge of tailgate is a plastic moulding so will not take to any
weight being hung off of it. The lower edge of the tailgate is ok though.
And the lower edge of the bumper centre section will not take kindly to
being used as a place to put a clamp/clip/hook around it either as it is
only a plastic/GRP panel and the lower/bottom egde doesn't fix to a
metal panel, so it is flexible.
TBH, the only way to go in respect of carrying bikes is to get a set of
roof bars, and then the fittings to carry the bike that attach to them.
The Smart ones are the neatest...just, and the only other make is Thule.
The Thule has adaptable feet so can be transferred from one vehicle to
another, unlike the Smart version.
Both compnaies do bike rack fittings for their bars. The Thule ones
should fit the Smart bars as the Thule roof boxes fit them ok.
But as already stated, a 'strap on' bike rack on the back of a ForFour
is a No-No unless (as Big Dunc has already said) you have a tow bar,
and you can then get/use a tow bar bike rack safely and with ease...
cos getting bikes up onto the roof bar can be a bit of a struggle !
