Is it a non-turbo 0.9 litre or a turbo 0.8 litre engine....?
either way it's still low mileage and will have some 'newness'
to 'burn off', and as mr.singh says, with the engine tucked away
in the somewhat confined rear of the car it will tend to get hot
back there. Also, there really is limited ventilation for the heat
of the engine and exhaust to escape from, so that side scoop is
an obvious point where some of it will 'baloon' out from when
stationary, especially just after shutting down, so it will tend to
smell stronger from there than anywhere else I would suspect.
Don't think that this is something to worry about as your ForFour
shares the same engine, running platform and more or less the
same bodyshell as the Renault Twingo, both of which would have
had extensive prototype and pre-production testing before release
to the public sector to ensure that they never got so hot as to exceed
the design temperatures. If they had, then considerable re-engineering
would have been undertaken to address that.
So...if your car has or had any overheating issues prior to your ownership;
including the associated smells that one would get with that; then one
would hope that it would have been identified, found and corrected at
the either the first or second service. But that's not to say you should
completely ignore what you feel isn't 'as it should be'...just be aware
that with all things mechanical they sometimes can and do go wrong
at the most inconvenient of times, so keep a wary eye/nose/ear out
till you get the 'All ok' at the next service and..after it's first MoT in 2019.