Oh my gosh. that's 4 goldies in only 4 US gallons (15L) of water. No wonder you're having trouble! you need about 4 liters per goldie.
So have you read up on cycling yet?
If not here is some good reading
http://www.marineland.com/articles/1firstthirty.aspyour goldies release ammonia. it's toxic. Until the bacteria in the filter that convert ammonia to nitrIte get up to speed, ammonia builds up and can kill them if you're not monitoring it and doing partial water changes and/or using an ammonia binder such as amquel to reduce ammonia levels. It takes a 2 or 3 weeks for the ammonia eaters to get up to speed.
The other set of bacteria in the filter convert nitrIte (still toxic) to nitrAte (less toxic) it takes them another 2 or 3 weeks to get up to speed. so again, monitoring your nitrIte levels and doing partial water changes to keep them low are essential. Salt can also be used to help make nitrIte less toxic. NitrIte enters thru the gills and binds up the red blood cells keeping them from being able to carry oxygen. Salt helps protect against nitrIte poisoning because it competes with nitrIte for uptake thru the gills. After you get it salted,
Can you get them into more water?
I'd change out about half of the water per day, making sure to match the temperature and dechlorinate.
I'd add 1 teaspoon of salt for every 4 liters of water. predissolve in tank water and make sure it disperses well when you add it. rock salt works fine. You can use table salt as long as it doesn't contain yellow prussiate of soda. Rock salt is cheap and works well. when you do partial water changes just add 1 tsp of salt per gallon for the next 3 weeks, then you can skip adding any more salt. Your partial water changes will then remove it over time.
I'd also cut back on feeding them. more food results in more ammonia.
Can you take some of your tank and tap water to a pet store and get them to test it for you? Have them tell you specific numbers for
ammonia,
nitrIte
nitrAte
pH
KH
GH