SWADHYAYA is the fourth Niyama.

Swadhyaya means Self study, Self analysis and Self awareness.  "O man, knoweth Thy self and the source from which though camest."  One must continually study, question, listen, contemplate and digest, whilst remaing 'open minded' and therefore grow in knowledge.  Think BIG and expand into the universe.  Once the mind is closed for the ego 'knows it all' stagnation develops, as it would if a flowing stream formed a puddle and then stopped. 

Where there is no growth then there is decay, stagnation, for nothing stays the same, everything changes. 

"You can never step into the same river twice!" Lao tse