Not a problem

All info is good to know.
This is actually not enough food to keep them healthy. Goldfish do not have a digestive system like other fish ( they lack a stomach for storing food for later digestion) so they need multiple feedings per day. The fish should be fed as much as they can eat in 1to2 minutes 3 to4 times per day at least.
This means we can't rule out internal injury caused by another fish.
This is where we run into a big problem. Goldfish, when fed properly, are very dirty fish so the more water they are in, the longer it takes for the water to go foul. Your tank is the recommended size for a single "fancy" (ie Oranda, Lionhead, Ryunkin, etc) goldfish. For the 4 you have, they should be in a tank at least 280 liters. The larger the better. As for the water change amounts and frequency, that 30% should be weekly or even twice weekly depending on the water parameters. Goldfish need clean water to stay healthy.
Unfortunately, "but the other fish are doing fine" is not how you judge the health of a tank. You have to consider that these goldfish are all man made creations so their genetics have been manipulated over time. This means that some fish may be genetically inferior to others and more susceptible to poor water quality than healthier lines. It's similar to the "Canary in the coal mine" where miners kept Canary birds in the mines to show them when the air quality was unhealthy. If the bird died, the men, still alive, needed to get out and get out fast. So just like the men in the mine doesn't mean the mine is safe, the other fish still doing okay isn't a sign that there is nothing wrong with the tank. I suggest you get a good liquid test kit ( API and Fritz Aquatics master kits are typically the ones most frequently used by hobbyists) and keep tabs on Ammonia and Nitrates. With goldfish, because of the amount of food that is necessary to feed, a healthy bed of nitrifying microbes will be converting all that ammonia from the "poo" eventually to nitrates. Some nitrates is safe, a lot of nitrates is not. You really don't want the level to be above 40 ppm unless you are keeping live plants that need the nitrates for food. For them, a nitrate level of 5 to 10 ppm is usually enough. Without plants, the lower the better.
So there are really a few items that may have caused this fish to get unhealthy that it will be hard to isolate which one(s) caused this. You now have a game plan on how to move forward to hopefully keep the others healthy. ( I would seriously consider a much bigger tank or rehoming at least 2 of the remaining 3 fish. Not to sound like a " Debbie Downer" but I don't hold much hope for the one with dropsy so I wouldn't rehome all of the remaining fish just yet. ) If the bigger tank is not an option for you, I'd plan on doing water changes at least 2 to 3 times a week to keep the other 3 fish healthy and don't add any more fish.
Hope this helps.