[darkfalz]

Pet stores will tell you anything to get the sale. If you realise your mistake, they get money again when you buy a bigger tank (or more fish to replace your dead ones).

That many fish in a 4 gallon is just not a good idea. The Betta by himself would be okay. A bigger tank will mean you only need to change the water once a week, and everyone is happier.

With Bettas, they often just get sick when you change their living conditions, especially if you don't acclimate them to their new water. It's just a mode they go on where they move very little and often their fins clamp. At this time all kinds of bacterias and things are trying to attack them because their immune system is shocked. This is why when I get a new Betta I always put him into his new water very gradually, and I put him first in an empty tank with no gravel and just the kind of water I use (PH, salinity etc.) so he can get used to that, then I move him into the full tank. Once he's used to the water and his immune system is strong, the things that live in the gravel etc. won't be a problem.