"Happy Birthday to my precious angel who has forever blessed my life," she wrote Saturday, along with a photo of her little one. "The happiness this boy brings me has no words to describe it."
The Keeping Up With the Kardashians star kicked off her son's fourth birthday with a fun week of Disney-themed activities, taking Mason to Disneyland Tuesday and to see Disney On Ice presents Rockin' Ever After Thursday.
While visiting the Happiest Place on Earth, the duo and baby Penelope took a ride on a merry-go-round, then hopped on Tomorrowland's Finding Nemo Submarine Voyage while they were at it.
And then it was time for some fun on the ice.
"No big deal just ice skated with Minnie and Mickey tonight! #disneyonice #beautyandthebeast," the mom of two posted on Instagram Thursday, along with a shot of her and Mason on skates.
Scott Disick also helped their son celebrate his special day, taking him out for breakfast Friday morning at the Marmalade Café in Calabasas.
Happy Birthday Mason!