ORIGINAL: Sliverflicker
Why not just put the 3 gallon container in the 5 gallon container, fill the 5 gallon one up with water and remove the 3 gallon one whichgives you 2 gallons in the 5 gallon, pour that into the 3 gallon and repeat the process. that will give you 2 gallons in each one so you dont hurt yourself carrying it, because you know if they were going to kill you over 4 gallons of water, they are gonna make you carry it!
Your idea sir is the most creative. In all the years I've asked this riddle, nobody's come up with that one. However, though the math works, the actual volumes wouldn't add up to 4 gallons. See, what if the containers are a half inch thick? You'd end up with less than 4 gallons and then you'd be dead. Well, at least your toil would be less a pint or so as you make your way to the firing squad right?
You know, it's odd that everyone can figure out how to do it both of the hard ways but can't the easy way if the containers were symmetrical. *shrugs