Kim Kardashian celebrates son on his 4th birthday

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He's regularly pictured with a wide smile on his face. And according to Kim Kardashian, Saint West's disposition is naturally sunny, with the reality star paying tribute to her oldest son as he turned four on Thursday.

'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,' wrote the proud mother.

She continued: 'You are so kind, loving and just so thoughtful and sweet! I am so happy today if your 4th birthday and We get to celebrate how much you’ve grown! Happy birthday my sweet sweet Sainty.'

And the doting 39-year-old added: '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?'