the tank may have had too much ammonia build up. if the tank was new then 4 mollies was too much at once. the temp could have been wrong. tropical fish need 75 degrees F. did they look ill? any white spots? could have been ick. is there a filter? maybe there is not enough oxygen in the water. is there an air stone?
fish owner of 13 years
To find out cause of death a profesional may nedd to perform an autopsy.