Fish Beginner in trouble

What you need to do is get rid of all of your fish and then do proper research before you get any fish. A 4 gallon tank is too small for any fish except maybe a betta fish, and I would not even recommend you get a betta fish in it. There is no point in trying to get a heater or something like that now because it won't solve your significant overstocking program. Instead, I would recommend just giving away the fish and then re-looking at your situation.