Firstly report the 'reputable' garden centre to your local consumer or trade standards org., wherever you live, for fraudulent representation.
You say only one is a goldfish.... does that mean the other 3 are tropical fish? Is there is heater in the tank? If they are just 1" or so you don't need a filter or if you have an airpump you can get away with an undergravel filter. It is always better to use a filter but as said above, regular PARTIAL water changes works good too.
If the goldfish gets too big make sure it is only put outside in the summer.