Do you use faucet water when refilling the tank? If so then you should be using the de-chlorination tabs, or the little bottle of liquid that treats the water. Also a 2' by 1' tank for 4 goldfish can be way too small, unless they're feeder fish, goldfish are naturally really dirty fish though, and especially with an un-established aquarium, they could make the water almost unbearable to live in within days. I used to have 5-6 goldfish, 2 comets, a shubunkin, and the rest fancy, and even in a 35 gallon (around 130 liters) it was a lot of upkeep to keep the water clean. But getting stuff like feeder shrimp, snails and aquatic plants helps a whole ton.
In my opinion though, if they're dieing hours after you change out the water, it could be a water treatment issue. Here are some tips:
- Be totally sure the water is either non chlorinated or is treated with special chemicals to make it fish safe
- Check the pH levels, ammonia, nitrites before the fish get introduced and after
- Leave your tank running continuously without cleaning and without fish, or only 1 goldfish for about a month to let it cycle
- Definitely upgrade to a larger tank if you want to keep over 3 goldfish, some can grow big too.