But while it doesn’t seem like that long ago since the young Cambridges were brought into the world, Prince George is due to turn nine later this year and Princess Charlotte is set to become seven-years-old in little over a week.
So when is Prince Louis’ birthday - and where is he in the line of succession to the throne?
Here’s everything you need to know.
When is Prince Louis’ birthday?
Prince Louis Arthur Charles of Cambridge is celebrating his birthday today - Saturday 23 April - just days after his grandmother, the Queen’s, birthday.
The young Prince has officially waved goodbye to his days as a toddler as this birthday is seeing him turn four-years-old.
He was born on St George’s Day 2018 in the private Lindo Wing of St Mary’s Hospital in Paddington, London, weighing 8lb 7oz.
As with his older brother and sister, Prince William and Kate Middleton have shared photographs of Louis to mark the occasion.
In the series of images taken on a Norfolk beach in April by the Duchess of Cambridge - the county’s coast being close to the Queen’s Sandringham estate - the young royal can be seen running barefoot and laughing.
When William and Kate are not at their London home - Kensington Palace - they spend most of their time at their Norfolk home of Anmer Hall.
One of the pictures shows Prince Louis holding a cricket ball, which is likely to be a reference to his great-grandfather the Duke of Edinburgh who was himself a keen cricketer and a fan of the sport.
Indeed, the late Duke twice served as president of the MCC - the official guardian of the game.
There is no official word of how Prince Louis is marking his big day, but it’s likely he will be celebrating with family and friends from Willcocks Nursery School - a London education establishment he’s been going to for the past year.
Where can you buy Prince Louis’ star jumper from?
Consciously or not, Kate Middleton has been a fashion influencer ever since she first came into the public eye.
So it’s no surprise that her children’s dress sense is pored over whenever images of them are released.
In Prince Louis’ birthday shots he is seen wearing a starry jumper on top of a shirt, with turquoise shorts.
It’s not clear where you can get hold of Louis’ knitwear, and it could be hard to find given Kate Middleton is known to recycle clothes Prince George has grown out of.
Where is Prince Louis in line of succession?
As the son of Prince William, Prince Louis is near the top of the line of succession to the throne.
Here is the line of succession as it currently stands:
- The Prince of Wales (aka Prince Charles)
- The Duke of Cambridge (aka Prince William)
- Prince George of Cambridge
- Princess Charlotte of Cambridge
- Prince Louis of Cambridge
Prince Harry, the Duke of Sussex, is listed as sixth in line to the throne, and his two children with Meghan Markle, the Duchess of Sussex - Archie and Lilibet - come after him.