The legend once said...
By the time Than Tru Troi (the God that holds up the sky in Vietnamese mythology) conquered the world, Nghe Than (the mythical half-lion-half-dog creature) was born and soon became the most devilish creature of all; yet it was then defeated by the Three Goddesses of Buddism and divided into two separate parts: the chaotic and the moderate

The chaotic part - the "Chaos" was turned into the Four Spirits of Hell
while the moderate part - the "Modesty" became the Four Guardians of Heaven.

Gradually, the original story of Nghe Than kept changing among generations and became a fague conception, which later on was defined as Fate.