You've got two jars, one of them fits exactly 5 litres, the other one fits exactly 3 litres. How could you get exactly 4 litres with these two jars? It is allowed to spoil some water.

Fill the 5 litre jar completely. Then fill the 3 litre jar using the 5 litre one completely. Now there are 2 litres left in the 5 litre jar. Now you empty the 3 litre jar and put the 2 litres from the 5 litre jar in the 3 litre jar. Then fill the five litre jar again compeletely. Finally fill the 3 litre jar completely using the 5 litre jar. One litre still fits, since there were already 2 litres in that jar. So then 4 litres remain in the 5 litre jar. You'll get exactly 4 litres in this way.