What kind are the other 3 and how big?
Is your tank cycled? How long has it been running and how long have you had the fish? Which filter are you using and how often and how do you clean it? Are you using a gravel vac? How much water do you change in a week and are you using a dechlorinator?
Do you have test kits for ammonia, nitrite, nitrate and ph? If not , then your local pet store can test it for you, but also take a sample of tap water so we can see if the ph has dropped or risen.
As mentioned, anything else wrong, such as red streaks through the fins, bruising on the body? Any spots?
What do you feed them on?