Fill in the 3L jug and pore the water in the 5L jug.
Again Fill the 3L jug and pore the water in 5L jug till the rim, Now you'll have 1L left in the 3L jug.
Empty the 5L jug and pore the 1L from 3L Jug to 5L.
Now again fill the 3L jug and pore that completely into 5L jug.
That will make a total of 4L in the 5L jug.