Answers:

Number 1
Fill the 5 litre can from the tap.
Empty the 5 litre can into the 3 litre can - leaving 2 litres in the 5 litre can.
Pour away the contents of the 3 litre can.
Fill the 3 litre can with the 2 litres from the 5 litre can - leaving 2 litres in the 3 litre can.
Fill the 5 litre can from the tap.
Fill the remaining 1 litre space in the 3 litre can from the 5 litre can.
Leaving 4 litres in the 5 litre can.

Number 2
Fill the 3 litre can from the tap.
Empty the contents of the 3 litre can into the 5 litre can.
Fill the 3 litre can from the tap.
Empty the contents of the 3 litre can into the 5 litre can. - Leaving the 5 litre can full and 1 litre in the 3 litre can.
Pour away the contents of the 5 litre can
Pour the 1 litre from the 3 litre can into the 5 litre can.
Fill the 3 litre can from the tap.
Empty the contents of the 3 litre can into the 5 litre can.
Leaving 4 litres in the 5 litre can.