Answer:

Fill the 5 gallon container and empty it into the 3 gallon contains so the latter is just full.  Now there is 2 gallons left in the 5 gallon container.  Empty the 3 gallon container and spill the 2 gallons into the 3 gallon container.  Fill the 5 gallon container and empty it so it just fills the 3 gallon container (you need one gallon to do this).  Now there are 4 gallons left in the 5 gallon container.

Another solution as pointed out by a Dr. Solina is to take the 3 gallon and fill it. Empty it into the 5 gallon. Now use the 3 gallon container again to fill the 5 gallon all the way. What remains in the 3 gallon container is one gallon. Now, empty the 5 gallon container and pour the one gallon into it. Just add the contents of a 3 gallon container into it to make 4 gallons.