you fill the 3 liter bottle up to the top, and pour the contents into the 5 liter bottle

then you fill 3 liter bottle again, and pour the contents into the 5 liter bottle until the 5 liter one is full

empty the 5 liter bottle, and pour the 1 liter in the 3 liter bottle into the 5 liter bottle

fill the 3 liter bottle again and pour that into the 1 liter already in the 5 liter bottle to get 4 liters of water

Then you bring the fox back, take the hen across the river, ...