you're going to need a bigger tank eventually even for just those 4! goldfish can get quite large. one once ate my dog. ok i lied, but seriously they do get big! if they are cramped that will lead to stress.
also, if your tank is brand new the bacteria that supports the nitrogen cycle isn't present, meaning your ammonia and nitrite levels can get dangerously high, which will also stress your fish, if it doesn't just kill them :( water changes on a very regular basis will help keep it in check, though :)
at any rate, stress makes fish very susceptible to illness, infections, and disease. get a good test kit and check your water parameters for nitrates, nitrites, and most importantly ammonia. but also check the sticky about ick, because hey! anything is possible! i don't want to rule OUT ick, ill leave that for a more senior member of the forum here (and thats just about everyone
) but getting a test kit and checking your parameters is going to be vital for the knowledgeable men and women here to help you out more :)