Consider 2 buckets one 4L and other 9L. : Bucket 1 (4L) and Bucket2 (9L)
Here is the step
First fill the 9L bucket fully. : 0 L and 9 L
Pour the water into 4L bucket. : 4 L and 5 L
Empty the 4L bucket. : 0 L and 5 L
Repeat this twice. : 4 L and 1 L
Now you will left with 1L water in the 9L bucket : 0 L and 1 L
Now pour this 1L into the 4L bucket : 1 L and 0 L
Refill the 9L bucket. : 1 L and 9 L
Now pour the water from 9L into the 4L bucket until it fills up. : 4 L and 6 L
Now you are left with 6 L water in the 9L bucket.