This is most likely a water quality issue, and also a bunch of fish that aren't even compatible..
To keep these 4 goldfish, you need a 150-200 liter tank MINIMUM. Each goldfish needs40-80 liters of water, you do the maths
A tank upgrade would be the best solution for now. Try finding a 200 liter and start cycling that while performing lots of smaller water changes every week on the current tank.