I would say now as hitting the 40k mark.
When I worked at VW HQ they all advised me 40k or 4 years.........which ever comes soonest, as this has now replaced the old text due to water pump failures and a few issues with belts mainly (covering their backs)
Just turning 60k on mine now, but the 4 year change interval is coming up sooner for me, so September time is will be the full monty for me.
All the garages have their own opinion weirdly, but I have stuck with what i was told by the HQ engineers.