"Happy Birthday Charlotte!" the couple commented underneath a Kensington Palace Instagram post.
Meghan made sure to add her own touch with a birthday cake emoji followed by a red balloon.
They signed off with, "Lots of love, H and M xo."
The sweet salutation was posted under Prince William and Kate, Duchess of Cambridge's post, which read, "The Duke and Duchess of Cambridge are very pleased to share three new photographs of Princess Charlotte ahead of her fourth birthday.
"The photographs were taken in April by The Duchess at Kensington Palace and at their home in Norfolk."
Just last month Harry and Meghan celebrated Charlotte's little brother Prince Louis' first birthday by posting a cute message under a similar series of photographs shared to Instagram. So modern!