Right. You might need to make a cuppa and sit down. 14 litres is not big enough for one goldfish, let alone four. Goldfish get very, very large and produce a lot of waste. They are not cheap or low maintenance pets. The shops assistant was talking utter, utter bollocks.
Goldfish do not grow to the size of their tank, rather they become stunted which causes all kinds of health problems. I'd advise against upgrading as they grow, for the reason that it's hard to tell whether or not they are becoming stunted until the damage is already done.
Here is a guide to how big they get and how much space they need .
14 litres is not big enough for any fish really, smaller tanks are often peddled at beginners but they are too difficult to maintain water quality to safely keep anything in. I'd go for a minimum of 45 litres (although something like 60-70 gives you loads more options).
I'd find a different shop too if you can, one that knows the head end of a fish from the tail end! If you let me know what county you're in I might be able to recommend somewhere.
Sorry for all the bad news :(