it seems they can reach a length of 8" and are extremely fast swimmers who will need the space as to not injure themselves. and since they need to be kept as either single fish, or in small shoals, you may not want to put them in a 4ft tank.
personally, im not sure i would put them in a 4ft wide tank, seeing as they wont be able to stop themselves quick enough if they have a burst of speed.
ive seen fast swimming fish that dont have enough swimming and then stopping room, and their faces are generally not pretty looking.