There are two ways to do that

• 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.

Or:

• 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.