They also said they would like to thank everyone for all the kind messages they have received ahead of the celebration.
Prince George had a beaming smile in the official photograph. He wore a blue, pinstriped shirt with a mandarin collar.
The photo has reminded us just how quickly kids grow up.
In July 2016 the photo released to mark his third birthday showed a younger looking George playing in the garden barefoot.
And in July 2015 when he was just a toddler, Prince George beamed at the camera with the Duke of Cambridge in the photo to mark his second birthday.
What a cutie!