Hmm..IMHO...don't rush to stock fish..be patient..slowly add in a fish week by week..this is to help u to monitor the fish u newly introduced to the tank..and find out its behaviour...
anyway..have u checked your nitrate level?..u said that u added 4 fishes after 2 weeks...it may be the cause of heavy bioload that your tank can't handle that cause the death of your fishes...try not to overstock too much..
get only expensive fishes after a month or so if you are confident but most experts recommend to let the tank cycle for 2 months before adding lifestock..
and also..in choosing fishes from LFS...be sure to monitor the fish closely..i spend up to half an hour in the shop just to watch the particular fish behaviour..before purchasing it..also..some people recommend to watch the fish a few days first in the LFS before purchasing it..
ask the LFS owner about the fish behaviour...if the bugger is eating, when was it brought into the LFS..also..check its tankmates condition..avoid purchasing it if you see any diseases on its fellow tankmate...
i am no experts but thats what i can advise..