AUGUST SOLUTION

Fill up the five litre bottle. Then use it to fill the three litre bottle so that two litres remain in the five litre jar. Empty the three litre jar and pour the two litres from the five litre jar into the three litre jar. Fill up the five litre jar again and use it to top up the three litre jar with the extra litre it needs. Four litres now remain in the five litre jar.

Alternative solution

Fill up the three litre jar and pour its contents into the five litre jar. Fill up the three litre jar again and use it to top up the five litre jar with the extra two litres it needs. Now empty the five litre jar, and pour the litre that is in the three litre jar into the empty five litre jar. Fill up the three litre jar again and pour its contents into the five litre jar. The five litre jar now contains exactly four litres.