Update 19 November: On Thursday evening, Prince William, Kate Middleton, Prince Harry and Meghan Markle made a surprise appearance at a private dinner for their Royal Foundation in central London.
For the occasion, the Duchess of Cambridge was seen in photographs, shared by singer Tom Walker on Instagram, dazzling in a bright red dress by Preen.
The £995 'Fenella' dress - which the royal previously wore in September 2016 for a reception at Government House in Victoria, British Columbia - is from the designer's AW17 collection.
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The design features an asymmetric sweetheart neckline, fitted waist, and full mid-length skirt.
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Meanwhile, Markle - who is pregnant with her first child - stunned in a black strapless gown with a fitted bodice and V-neck cut at the bust.
It is widely believed the Duchess of Sussex's dress was designed by close friend Roland Mouret. She teamed the look with a dark-coloured clutch bag.
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The mother-to-be has frequently worn designs by Mouret, including a grey fitted dress during her two-day tour of Ireland and the night before her wedding at Cliveden House.
From Princess Eugenie's wedding to the commemorative events marking Remembrance Sunday, we've seen the Cambridges and the Sussexes at several engagements in recent weeks.
Following Prince Charles' 70th birthday celebrations on Wednesday, the royal foursome - commonly now known as the 'Fab Four' - stepped out together once again on Thursday evening, but this time it was for a private appearance.
Last night, William, Kate, Harry and Meghan attended a dinner for their Royal Foundation in central London.
A note from the Court Circular, the official record of royal engagements, confirmed their outing:
'The Duke and Duchess of Cambridge and The Duke and Duchess of Sussex, Joint Patrons, the Royal Foundation of The Duke and Duchess of Cambridge and The Duke and Duchess of Sussex, this evening attended a Dinner at Victoria House, Bloomsbury Square, London WC1.'
Earlier in the day, William and Kate had been at BBC Broadcasting House, where the Duke gave a powerful speech highlighting the dangers of social media as part of the Royal Foundation's Cyberbullying Taskforce.
The Royal Foundation was set up to look after the princes charitable projects in 2009. The Duchess of Cambridge became a patron shortly after she married William in 2011, with Meghan joining the trio earlier this year.
The Foundation's current initiatives include Heads Together, the Invictus Games and more recently Early Intervention for Children and Families, launched by Kate in March, and Together, a cookbook created by Meghan to support the Grenfell Tower community.
Speaking at the first forum for the Foundation back in February, Meghan said she was keen to 'hit the ground running' with her charity interests, which are focused on supporting women.
'Right now with so many campaigns like MeToo and Time’s Up there’s no better time to continue to shine a light on women feeling empowered and people supporting them,' she said.
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Her first charity initiative - the Together cookbook - has already been a hit. According to People, it sold 11,000 copies in the U.K. during the first week of its launch, and reached number six on Amazon's list of most-sold books in the U.S.
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