You're looking at about 165 pounds of cargo for four cans (a little over 8lbs per gallon plus weight of the cans) plus the weight of the rack. All told, you're probably in the 250-300 pound range. They should hold up well, especially if you can evenly balance the weight on each side. Putting more weight on one side may cause some bending of the rack if the weight bounces in transit.
I see those sort of racks in use quite a bit, so I'm guessing they hold up well. Be mindful of having the cans exposed to direct sun while in transit... Don't drive too far on a really hot day.