2 m long 4" (100mm) diameter aluminium foil tubes are readily available in all good hardware and diy stores.
Plastic bending pipes are also available but are usually used for vent slates etc. Other contributers may recommend insulated pipes - which can be expensive but can prevent condensation of air in pipe. I suppose it depends on position of pipe.
If this pipe is in a ceiling then consider packing around pipe with fibre glass or Rockwool insulation - between joists, if possible.
It should be possible to get a "downsizer" to fit to existing 6" pipe. Connect 4" foil to 4" side of downsizer and connect to 4"extractor fan / downsizer (4" to 6").
you could fit 6" to 4" downsizer to fan, fit foil pipe and insert same into 6" pipe. Use spray foam to seal gap between pipes; or cover connection of pipes in 50mm aluminium tape - good overlap = air tight seal!