Put 4L + 4L of water in a 9L container. 9L container now contains 8L of water. From 4L container, put 1L in the 9L container. 4L container now contains 3L. Empty the 9L container. Put 3L from the 4L container into 9L container. Put 4L water into 9L container. This makes 7L (4L + 3L) water in the 9L container. Put 2L water from 4L container into 9L container. 4L container now contains 2L of water. Empty the 9L container again. Put 2L from the 4L into the 9L container. Put 4L into the 9L container. That makes it 6L (2L + 4L) in a 9L container!!