To measure 4 gallons using  3 gallons and 5 gallons:
1. Fill the 5 gallons Jar.
2. Pour the water from 5 gallons jar to 3 gallons Jar. So 5 gallon will have 2 gallons left.
3. Throw the water in 3 gallons Jar and pour the remaining 2 gallons from 5 to 3 jar.So 3 has 2 gallons of water now and can accommodate 1 gallon more.
4. Again fill the 5 gallons jar.
5. Now fill the 3-gallon jar with 5 gallons water.
6.  Since 3-gallon jar can accommodate only one gallon, the remaining gallons of water in 5 gallons jar is 4 gallons of water.

To measure 4-gallon water using 3 and 4-gallon jar. Just use the 4-gallon jar to measure 4 gallon of water