• Fill the 3L jug.
• Pour the 3L jug into the 5L jug. The 5L jug now has exactly 3L of water.
• Fill the 3L jug again.
• Pour the 3L jug into the 5L jug until it is full. There is now 5L of water in the 5L jug, and 1L remaining in the 3L jug.
• Pour out the 5L jug.
• Pour the 1L of water from the 3L jug into the 5L jug.
• Fill the 3L jug and pour it into the 5L jug. There is now exactly 4L of water in the 5L jug.

Might as well add my own solution:

• Fill the 5L jug
• Pour the 5L jug into the 3L jug until the 3L jug is full
• Now the 5L jug contains 2L
• Empty the 3L jug
• Pour the 5L jug into the 3L jug
• Now the 3L jug contains 2L
• Fill the 5L jug
• Pour the 5L jug into the 3L jug until the 3L jug is full
• Now the 5L jug contains 4L

• Fill 5l
• Use 5l to fill 3l
• Ditch 3l
• Your 5l now contains 2l
• Move 2l from 5l to 3l
• You now have 0l and 2l
• Fill 5l again full and fill 3l to the top Now you have 4l in 5l and 3l in 3l

Overall whoever sets this task should be shot as you just wasted a crap ton of water instead of using a measuring jug...

If the 4L can be poured somewhere else, it gets even easier:

• fill 5L can
• pour from 5L into 3L. 5L now has 2L left. Pour that somewhere.
• repeat above once more to add another 2L.

1. Full the 5L jug, then pour it into to 3L jug. Now there's 2L in the 5L container, and the 3L jug is full.

2. Dump out the 3L jug and pour the 2L into it. Now the 5L jug is empty.

3. Fill the 5L jug again, then pour into the 3L jug until it's full. There's only room for 1L, so that leaves 4L in the bigger jug. Dump out the small jug, é voila!