First, fill 3Lt bottle and pour 3 litres into 5Lt bottle.
Again fill the 3Lt bottle. Now pour 2 litres into the 5Lt bottle until it becomes full.
Now empty 5Lt bottle.
Pour remaining 1 litre in 3Lt bottle into 5Lt bottle.
Now again fill 3Lt bottle and pour 3 litres into 5Lt bottle.
Now you have 4 litres in the 5Lt bottle. That’s it.