Kristin Scott Thomas, who played Fiona, was there too.
In the 1994 film, her love for Charles was unrequited, but Emma Freud, Curtis's wife, shared a tantalising picture from the set of Grant and Scott Thomas appearing to share an intimate moment.
Curtis said: "We're all definitely older - I suspect no wiser. It's been really enjoyable working out what's happened to all the characters. "Now they get back together for the fifth wedding where, as usual, not everything will go as planned."
The one-off special, directed by Mike Newell, will be shown during the Red Nose Day broadcast in Britain on March 15.
Four Weddings and a Funeral propelled Curtis and Grant to international success.
In the past, Grant said he feared the film would be a disaster: "Four Weddings was shot in 36 days in a blind panic with the director hurling tea cups at the wall. We thought it didn't work and that we would all have to emigrate to Peru ..."