Harry and Meghan, who are awaiting the birth of their own child any day now, replied to a Kensington Palace Instagram post featuring new photographs of the fourth in line to the throne.
Signing off as “H and M”, the couple wrote to their niece: “Happy Birthday Charlotte! Lots of love, H and M xo” - along with emojis of a chocolate birthday cake and a red balloon.
Clarence House also wished the princess a “very Happy Birthday” on Twitter, as did the Royal Family.
Kensington Palace said via Instagram: “Thank you for the lovely messages for Princess Charlotte’s fourth birthday!”
Three pictures of Charlotte, taken by her mother, the Duchess of Cambridge, at Kensington Palace and the family’s Norfolk home, Anmer Hall, were released to mark the princess turning four on Thursday.
In the first picture, Charlotte is pictured in a floral summer Betsy dress by Trotters as she sits cross-legged on a patch of grass at the palace.
In the other two images, taken at Anmer Hall, the Queen’s great-granddaughter is pictured running and smiling as she holds a flower, and sitting on a fence beaming at the camera.
The youngster, whose full title is Her Royal Highness Princess Charlotte Elizabeth Diana of Cambridge, was born at the Lindo Wing at St Mary’s Hospital in Paddington at 8.34am on May 2 2015, weighing 8lb 3oz.
She is the second child of the Duke and Duchess of Cambridge, and a sister to Prince George and Prince Louis.
Kate, one of the world’s most photographed people, often goes behind the lens and has released her own pictures on a number of occasions.
Just last week, the 37-year-old duchess released snaps of her youngest son, Louis, to mark his first birthday.
Kate is a keen and accomplished amateur photographer who has lifetime honorary membership of the Royal Photographic Society which recognised her “talent and enthusiasm”.
She bucked tradition when she took the official photographs of newborn daughter Charlotte in 2015.
The photos of Charlotte showed the young princess being cradled by big brother George at William and Kate’s Norfolk home.
Kate, a history of art graduate, has released pictures to mark other milestones in her children’s lives, including George and Charlotte’s first days at nursery school and Charlotte’s first birthday.