Males are very territorial, and will try kill another male if it is in it\'s territory. In the wild, the other fish is normally able to swim away. In a fish tank, not so much. Females are also very territorial and need a large sized tank if you want to keep them together. You also need more than 4 if you want to balance the aggression out, setting up a female 'sorority' as it\'s called, is very challenging and takes lots of research. Like I said, the forum link I posted is very knowledgeable, so anyone interested should ask questions and research there. Bettas are normally fine with other tropical fish though, but always check for compatibility before you add a new fish. ALWAYS do your research and good luck!