You can, but it would be best to introduce them to the tank all at the same time. You need to make sure you know what gender you are getting though, because it should be 2 - 3 females per male, and you must have a 50G or larger tank if you plan on keeping more than one male. So i'd say that if you have a male right now, then only look for females. If you have a female, then get one male and one female. Or two females if you want four of them, but i would advise against it.
How many other fish are in your tank right now?