Kim Kardashian and Kanye West's eldest son Saint West is celebrating his fourth birthday today!

That's right, it's been four years since the superstar couple welcomed Saint into the world, their second child together. The Keeping Up With the Kardashians star announced her baby boy's arrival in Dec. 2015, telling her fans, "KANYE AND I WELCOME OUR BABY BOY!"

Kim also added in her message, "Mother and son are doing well."

Now, four years later, Kim is sharing a sweet message to her son on his special day. Taking to social media to post a photo of a smiling Saint, the KKW beauty founder wrote, "I have no words to describe how much I love your smile and those curls of yours Saint! You bring so much joy into my soul. You are so kind, loving and just so thoughtful and sweet!"

Kim went on to write that she's "so happy" it's Saint's birthday and that they get to celebrate how much he's grown.

"Happy birthday my sweet sweet Sainty," Kim continued. "(When I say to him my sweet sweet Sainty, he says back to me my sweet sweet sweet sweet Momma, the best momma in the world and the only momma I ever want! HOW CAN YOU NOT MELT AT HIS SWEETNESS)."

After seeing Kim's post, Kris Jenner commented, "MELT [heart eye emojis]."

Khloe Kardashian also commented on the post about her nephew's birthday, writing, "He's honestly one of the sweetest humans I know on this earth!!!!! I'm melting by your caption and his face [heart eyes emojis]."

Kim and Kanye—who are also parents to North West, Chicago West and Psalm West—recently returned from a family trip to Japan for Thanksgiving. The Wests were joined on the vacation by Kourtney Kardashian and her three kids Mason Disick, Penelope Disick and Reign Disick.

There's much to celebrate in the celeb family in the upcoming weeks, in addition to the holidays, Mason and Reign will be celebrating their birthdays on Dec. 14!

Keeping Up With the Kardashians returns spring 2020, only on E!

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