"Happy birthday to my precious angel who has forever blessed my life. The happiness this boy brings me has no words to describe it," the 34-year-old posted with the photograph on Instagram, reports eonline.com.
The "Keeping Up With the Kardashians" star started her son's fourth birthday celebration with a fun trip to Disneyland.
Kourtney gave birth to Mason in December 2009 with her long-time partner Scott Disick. The couple also has a daughter, Penelope.
--IANS (Posted on 15-12-2013)